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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

akutz/go,Cofyc/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,Cofyc/go,polyverse-security/go,codestation/go,akutz/go,akutz/go,josharian/go,josharian/go,CAFxX/go,golang/go,akutz/go,codestation/go,CAFxX/go,Cofyc/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,Cofyc/go,nawawi/go,codestation/go,polyverse-security/go,CAFxX/go,codestation/go,akutz/go,polyverse-security/go,josharian/go,christopher-henderson/Go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,josharian/go,cloudflare/go,Cofyc/go,codestation/go,golang/go,polyverse-security/go,cloudflare/go,josharian/go,cloudflare/go,Cofyc/go,akutz/go,cloudflare/go,josharian/go,codestation/go,Cofyc/go,akutz/go,cloudflare/go,nawawi/go,polyverse-security/go,nawawi/go,codestation/go,Cofyc/go,josharian/go,josharian/go,nawawi/go,polyverse-security/go,polyverse-security/go,golang/go,codestation/go,golang/go,golang/go,polyverse-security/go,golang/go,golang/go