all: reduce init-time CPU & memory usage

Tracking bug to reduce init-time CPU & memory usage.

Previously:

https://go-review.googlesource.com/c/go/+/127075 - html: lazily populate Unescape tables https://go-review.googlesource.com/c/net/+/127275 - http2/hpack: lazily build huffman table on first use

Open:

#26752 - x/text/unicode/norm: reduce init-time memory usage

The program https://play.golang.org/p/9ervXCWzV_z is useful to find offenders:

$ go build all.go
$ GODEBUG=memprofilerate=1  ./all
346768
$ go tool pprof /tmp/all.mem.prof 
(pprof) top 50
Showing nodes accounting for 286.24kB, 88.87% of 322.09kB total
Dropped 228 nodes (cum <= 1.61kB)
Showing top 50 nodes out of 133
      flat  flat%   sum%        cum   cum%
   37.03kB 11.50% 11.50%    37.03kB 11.50%  runtime.procresize
      32kB  9.94% 21.43%       32kB  9.94%  hash/crc64.makeSlicingBy8Table
   21.39kB  6.64% 28.07%    21.39kB  6.64%  vendor/golang_org/x/text/unicode/norm.init
   19.98kB  6.20% 34.28%    19.98kB  6.20%  strings.NewReplacer
   18.95kB  5.88% 40.16%    18.97kB  5.89%  encoding/xml.init
   12.17kB  3.78% 43.94%    12.17kB  3.78%  unicode.init
   10.92kB  3.39% 47.33%    10.92kB  3.39%  regexp/syntax.appendRange
   10.61kB  3.29% 50.63%    10.61kB  3.29%  vendor/golang_org/x/net/http2/hpack.(*headerFieldTable).addEntry (inline)
   10.38kB  3.22% 53.85%    12.62kB  3.92%  sync.(*Map).LoadOrStore
    9.38kB  2.91% 56.76%     9.38kB  2.91%  runtime.malg
    9.17kB  2.85% 59.61%     9.22kB  2.86%  html/template.init
    8.70kB  2.70% 62.31%    69.47kB 21.57%  net/http.init
    6.84kB  2.12% 64.43%     6.84kB  2.12%  regexp/syntax.(*compiler).inst (inline)
    6.12kB  1.90% 66.34%    13.12kB  4.07%  runtime.allocm
    5.25kB  1.63% 67.97%     5.25kB  1.63%  math/rand.NewSource
    5.06kB  1.57% 69.54%     5.47kB  1.70%  net/http.init.0
    4.03kB  1.25% 70.79%     4.22kB  1.31%  net/http.init.2
    3.94kB  1.22% 72.01%     3.95kB  1.23%  debug/dwarf.init
    3.72kB  1.15% 73.17%     3.72kB  1.15%  errors.New
    3.62kB  1.13% 74.29%    12.13kB  3.77%  encoding/gob.validUserType
    3.62kB  1.13% 75.42%     3.62kB  1.13%  regexp/syntax.init
    2.91kB   0.9% 76.32%     2.91kB   0.9%  go/types.(*Scope).Insert
    2.70kB  0.84% 77.16%    26.97kB  8.37%  encoding/gob.init
    2.69kB  0.83% 77.99%     2.69kB  0.83%  flag.(*FlagSet).Var
    2.59kB  0.81% 78.80%     2.59kB  0.81%  net/mail.init.0
    2.25kB   0.7% 79.50%     2.25kB   0.7%  compress/flate.(*huffmanEncoder).generate
    2.25kB   0.7% 80.20%     2.25kB   0.7%  net/textproto.init.0
    2.09kB  0.65% 80.85%     2.09kB  0.65%  image/jpeg.(*huffmanLUT).init
    1.89kB  0.59% 81.43%     1.89kB  0.59%  compress/flate.newHuffmanEncoder
    1.80kB  0.56% 81.99%     1.80kB  0.56%  go/types.NewTypeName
    1.77kB  0.55% 82.54%     1.77kB  0.55%  text/template/parse.(*Tree).newText (inline)
    1.75kB  0.54% 83.08%     4.38kB  1.36%  runtime.mcommoninit
    1.66kB  0.51% 83.60%     8.84kB  2.74%  go/types.init
    1.64kB  0.51% 84.11%    28.67kB  8.90%  go/doc.init
    1.59kB  0.49% 84.60%     3.57kB  1.11%  encoding/gob.bootstrapType
    1.55kB  0.48% 85.08%     2.98kB  0.93%  net.init
    1.39kB  0.43% 85.51%    32.34kB 10.04%  go/build.init
    1.17kB  0.36% 85.88%     3.09kB  0.96%  text/template.init
    1.16kB  0.36% 86.24%     3.55kB  1.10%  encoding/gob.newTypeObject
    1.06kB  0.33% 86.57%    14.55kB  4.52%  encoding/gob.RegisterName
    1.02kB  0.32% 86.88%     4.57kB  1.42%  encoding/gob.buildTypeInfo
    0.94kB  0.29% 87.17%    21.06kB  6.54%  regexp.compile
    0.91kB  0.28% 87.45%    16.88kB  5.24%  encoding/gob.init.1
    0.75kB  0.23% 87.69%     1.84kB  0.57%  html/template.New
    0.69kB  0.21% 87.90%     2.13kB  0.66%  reflect.(*rtype).ptrTo
    0.67kB  0.21% 88.11%     7.20kB  2.24%  crypto/tls.init
    0.67kB  0.21% 88.32%    22.42kB  6.96%  vendor/golang_org/x/net/http/httpguts.init
    0.61kB  0.19% 88.51%    14.49kB  4.50%  archive/tar.init
    0.61kB  0.19% 88.70%     5.40kB  1.68%  net/http/pprof.init
    0.56kB  0.17% 88.87%     7.81kB  2.43%  runtime.systemstack

(pprof) top --cum 50
Showing nodes accounting for 222.77kB, 69.16% of 322.09kB total
Dropped 228 nodes (cum <= 1.61kB)
Showing top 50 nodes out of 133
      flat  flat%   sum%        cum   cum%
    0.09kB 0.029% 0.029%   268.74kB 83.44%  runtime.main
         0     0% 0.029%   266.68kB 82.80%  main.init
         0     0% 0.029%    70.38kB 21.85%  expvar.init
    8.70kB  2.70%  2.73%    69.47kB 21.57%  net/http.init
         0     0%  2.73%    37.41kB 11.61%  runtime.rt0_go
   37.03kB 11.50% 14.23%    37.03kB 11.50%  runtime.procresize
         0     0% 14.23%    37.03kB 11.50%  runtime.schedinit
    1.39kB  0.43% 14.66%    32.34kB 10.04%  go/build.init
         0     0% 14.66%       32kB  9.94%  hash/crc64.init
      32kB  9.94% 24.60%       32kB  9.94%  hash/crc64.makeSlicingBy8Table
    1.64kB  0.51% 25.10%    28.67kB  8.90%  go/doc.init
    2.70kB  0.84% 25.94%    26.97kB  8.37%  encoding/gob.init
    0.67kB  0.21% 26.15%    22.42kB  6.96%  vendor/golang_org/x/net/http/httpguts.init
         0     0% 26.15%    21.75kB  6.75%  vendor/golang_org/x/net/idna.init
   21.39kB  6.64% 32.79%    21.39kB  6.64%  vendor/golang_org/x/text/unicode/norm.init
         0     0% 32.79%    21.06kB  6.54%  regexp.Compile
         0     0% 32.79%    21.06kB  6.54%  regexp.MustCompile
    0.94kB  0.29% 33.09%    21.06kB  6.54%  regexp.compile
   19.98kB  6.20% 39.29%    19.98kB  6.20%  strings.NewReplacer
   18.95kB  5.88% 45.17%    18.97kB  5.89%  encoding/xml.init
    0.91kB  0.28% 45.46%    16.88kB  5.24%  encoding/gob.init.1
         0     0% 45.46%    14.55kB  4.52%  encoding/gob.Register
    1.06kB  0.33% 45.79%    14.55kB  4.52%  encoding/gob.RegisterName
         0     0% 45.79%    14.55kB  4.52%  encoding/gob.registerBasics
    0.61kB  0.19% 45.97%    14.49kB  4.50%  archive/tar.init
         0     0% 45.97%    13.70kB  4.25%  fmt.init
    6.12kB  1.90% 47.88%    13.12kB  4.07%  runtime.allocm
         0     0% 47.88%    12.72kB  3.95%  regexp/syntax.Parse
   10.38kB  3.22% 51.10%    12.62kB  3.92%  sync.(*Map).LoadOrStore
         0     0% 51.10%    12.17kB  3.78%  reflect.init
   12.17kB  3.78% 54.88%    12.17kB  3.78%  unicode.init
         0     0% 54.88%    12.13kB  3.77%  encoding/gob.userType
    3.62kB  1.13% 56.00%    12.13kB  3.77%  encoding/gob.validUserType
         0     0% 56.00%    11.94kB  3.71%  runtime.mstart
         0     0% 56.00%    11.50kB  3.57%  regexp/syntax.(*parser).parseClass
         0     0% 56.00%    11.25kB  3.49%  runtime.newm
   10.92kB  3.39% 59.39%    10.92kB  3.39%  regexp/syntax.appendRange
         0     0% 59.39%    10.81kB  3.36%  vendor/golang_org/x/net/http2/hpack.init
    0.05kB 0.015% 59.41%    10.75kB  3.34%  vendor/golang_org/x/net/http2/hpack.newStaticTable
   10.61kB  3.29% 62.70%    10.61kB  3.29%  vendor/golang_org/x/net/http2/hpack.(*headerFieldTable).addEntry (inline)
         0     0% 62.70%    10.50kB  3.26%  regexp/syntax.(*parser).parseUnicodeClass
         0     0% 62.70%    10.50kB  3.26%  regexp/syntax.appendTable
    9.38kB  2.91% 65.61%     9.38kB  2.91%  runtime.malg
    9.17kB  2.85% 68.46%     9.22kB  2.86%  html/template.init
         0     0% 68.46%     9.05kB  2.81%  go/importer.init
         0     0% 68.46%     9.05kB  2.81%  go/internal/gccgoimporter.init
    1.66kB  0.51% 68.97%     8.84kB  2.74%  go/types.init
    0.05kB 0.015% 68.99%     8.80kB  2.73%  mime/multipart.init
    0.56kB  0.17% 69.16%     7.81kB  2.43%  runtime.systemstack
         0     0% 69.16%     7.69kB  2.39%  html/template.(*Template).Parse