os: use handle based APIs to read directories on windows

os: use handle based APIs to read directories on windows

This CL updates File.readdir() on windows so it uses
GetFileInformationByHandleEx with FILE_ID_BOTH_DIR_INFO
instead of Find* APIs. The former is more performant because
it allows us to buffer IO calls and reduces the number of system calls,
passing from 1 per file to 1 every ~100 files
(depending on the size of the file name and the size of the buffer).

This change improve performance of File.ReadDir by 20-30%.

name        old time/op    new time/op    delta
ReadDir-12     562µs ±14%     385µs ± 9%  -31.60%  (p=0.000 n=9+9)

name        old alloc/op   new alloc/op   delta
ReadDir-12    29.7kB ± 0%    29.5kB ± 0%   -0.88%  (p=0.000 n=8+10)

name        old allocs/op  new allocs/op  delta
ReadDir-12       399 ± 0%       397 ± 0%   -0.50%  (p=0.000 n=10+10)

This change also speeds up calls to os.SameFile when using FileStats
returned from File.readdir(), as their file ID can be inferred while
reading the directory.

Change-Id: Id56a338ee66c39656b564105cac131099218fb5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/452995
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>