io/fs: add FS, File, ReadDirFile; move DirEntry from os

io/fs: add FS, File, ReadDirFile; move DirEntry from os

These are the core interfaces for the io/fs design.
See #41190 and https://golang.org/s/draft-iofs-design for details.

DirEntry was left behind in the previous move from os
but is needed for ReadDirFile, so it moves in this commit.

Also apply a couple comment changes suggested in
the review of CL 261540.

For #41190.

Change-Id: I087741545139ed30b9ba5db728a0bad71129500b
Reviewed-on: https://go-review.googlesource.com/c/go/+/243908
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>