regexp: avoid alloc in QuoteMeta when not quoting

regexp: avoid alloc in QuoteMeta when not quoting

Many users quote literals in regular expressions just in case.
No need to allocate then.

Note: Also added benchmarks for quoting and not quoting.

	name             old time/op    new time/op     delta
	QuoteMetaAll-4      629ns ± 6%      654ns ± 5%    +4.01%        (p=0.001 n=20+19)
	QuoteMetaNone-4    1.02µs ± 6%     0.20µs ± 0%   -80.73%        (p=0.000 n=18+20)

	name             old speed      new speed       delta
	QuoteMetaAll-4   22.3MB/s ± 6%   21.4MB/s ± 5%    -3.94%        (p=0.001 n=20+19)
	QuoteMetaNone-4  25.3MB/s ± 3%  131.5MB/s ± 0%  +419.28%        (p=0.000 n=17+19)

	name             old alloc/op   new alloc/op    delta
	QuoteMetaAll-4      64.0B ± 0%      64.0B ± 0%      ~     (all samples are equal)
	QuoteMetaNone-4     96.0B ± 0%      0.0B ±NaN%  -100.00%        (p=0.000 n=20+20)

	name             old allocs/op  new allocs/op   delta
	QuoteMetaAll-4       2.00 ± 0%       2.00 ± 0%      ~     (all samples are equal)
	QuoteMetaNone-4      2.00 ± 0%      0.00 ±NaN%  -100.00%        (p=0.000 n=20+20)

Change-Id: I38d50f463cde463115d22534f8eb849e54d899af
Reviewed-on: https://go-review.googlesource.com/31395
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>