html: speed up UnescapeString

html: speed up UnescapeString

Add benchmarks for for sparsely escaped and densely escaped strings.
Then speed up the sparse unescaping part heavily by using IndexByte and
copy to skip the parts containing no escaping very fast.

Unescaping densely escaped strings slower because of
the new function call overhead. But sparsely encoded strings are seen
more often in the utf8 enabled web.

We win part of the speed back by looking up entityName differently.

	benchmark                  old ns/op    new ns/op    delta
	BenchmarkEscape                31680        31396   -0.90%
	BenchmarkEscapeNone             6507         6872   +5.61%
	BenchmarkUnescape              36481        48298  +32.39%
	BenchmarkUnescapeNone            332          325   -2.11%
	BenchmarkUnescapeSparse         8836         3221  -63.55%
	BenchmarkUnescapeDense         30639        32224   +5.17%

Change-Id: If606cb01897a40eefe35ba98f2ff23bb25251606
Reviewed-on: https://go-review.googlesource.com/10172
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>