proposal: io: pass WriterTo through Pipe

[hanwen-flow]

Proposal Details

It seems that io.PipeReader could implement WriterTo, such that connecting two files using a io.Pipe (a pattern common inside docker and friends) would use copy_file_range.

I coded up a prototype in https://go-review.googlesource.com/c/go/+/664395. This would add PipeReader.WriteTo.

[Jorropo]

I agree