The html package uses some specific code to escape special characters.
Actually, the strings.Replacer can be used instead, and is much more
efficient. The converse operation is more complex but can still be
slightly optimized.

Credits to Ken Bloom (kabloom@google.com), who first submitted a
similar patch at https://codereview.appspot.com/141930043

Added benchmarks and slightly optimized UnescapeString.

benchmark                   old ns/op     new ns/op     delta
BenchmarkEscape-4           118713        19825         -83.30%
BenchmarkEscapeNone-4       87653         3784          -95.68%
BenchmarkUnescape-4         24888         23417         -5.91%
BenchmarkUnescapeNone-4     14423         157           -98.91%

benchmark                   old allocs     new allocs     delta
BenchmarkEscape-4           9              2              -77.78%
BenchmarkEscapeNone-4       0              0              +0.00%
BenchmarkUnescape-4         2              2              +0.00%
BenchmarkUnescapeNone-4     0              0              +0.00%

benchmark                   old bytes     new bytes     delta
BenchmarkEscape-4           24800         12288         -50.45%
BenchmarkEscapeNone-4       0             0             +0.00%
BenchmarkUnescape-4         10240         10240         +0.00%
BenchmarkUnescapeNone-4     0             0             +0.00%

Fixes #8697

Change-Id: I208261ed7cbe9b3dee6317851f8c0cf15528bce4
Reviewed-on: https://go-review.googlesource.com/9808
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>