io: Add benchmarks for CopyN

io: Add benchmarks for CopyN

Copied from CL 60630

Current results:
name          time/op
CopyNSmall-4  2.20µs ±90%
CopyNLarge-4   136µs ±56%

name          alloc/op
CopyNSmall-4  1.84kB ±21%
CopyNLarge-4   128kB ±10%

name          allocs/op
CopyNSmall-4    1.00 ± 0%
CopyNLarge-4    1.00 ± 0%

Change-Id: If08c0132a773e936c9f61bff96e0aabf58006d31
Reviewed-on: https://go-review.googlesource.com/64932
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>