NIOFileSystem: No API to get the on-disk size

NIOFileSystem's FileInfo.size always reports the "apparent size" which is the actual file size (du -Ams report similar stuff) and that's good & correct.

It would be great to also get an .onDiskSize which reports back what's actually used on disk. This is important to account for two things

1. holes in files

# let's create a 1GB file that's mostly a hole
$ rm -f /tmp/hole && dd if=/dev/urandom bs=1 count=1 seek=$((1024*1024*1024)) of=/tmp/hole
1+0 records in
1+0 records out
1 bytes transferred in 0.000084 secs (11905 bytes/sec)

# apparent size is 1GB, fine
$ du -Ah /tmp/hole
1.0G	/tmp/hole

# but it only occupies 4 kB on disk
$ du -h /tmp/hole
4.0K	/tmp/hole

2. APFS compressed files

The implementation is just S_BLKSIZE * st_blocks where st_blocks is from stat. And please note it is S_BLKSIZE (it's 512, I think required by posix) and NOT stat.st_blksize (which can be 4kB or 1MB, depending on the fs), that's the preferred I/O size or something and must be ignored.