Use a 64 byte array to avoid an allocation on the assumption that
most url escaping is performed on short strings. Also adds a fast
path for escaping strings whose only replacements are spaces which
is common in query components.

Adds benchmarks for QueryEscape, PathEscape, QueryUnescape and
PathUnescape but no optimizations are include for the unescape functions
so I don't include those benchmark results here.

Reduces allocations by 10% in the existing String benchmark with a
modest performance increase.

name               old time/op    new time/op    delta
QueryEscape/#00-8    64.6ns ± 1%    43.8ns ± 0%  -32.14%  (p=0.000 n=9+9)
QueryEscape/#01-8     276ns ± 3%     249ns ± 0%   -9.62%  (p=0.000 n=10+7)
QueryEscape/#02-8     176ns ± 2%     155ns ± 3%  -12.21%  (p=0.000 n=10+10)
QueryEscape/#03-8     388ns ± 1%     362ns ± 0%   -6.55%  (p=0.000 n=10+8)
QueryEscape/#04-8    2.32µs ± 2%    2.27µs ± 2%   -2.26%  (p=0.001 n=10+10)
PathEscape/#00-8     78.0ns ± 3%    63.4ns ± 1%  -18.69%  (p=0.000 n=10+10)
PathEscape/#01-8      276ns ± 2%     260ns ± 0%   -6.01%  (p=0.000 n=10+10)
PathEscape/#02-8      175ns ± 0%     153ns ± 0%  -12.53%  (p=0.000 n=8+10)
PathEscape/#03-8      389ns ± 2%     361ns ± 0%   -7.21%  (p=0.000 n=10+9)
PathEscape/#04-8     2.30µs ± 2%    2.27µs ± 1%   -1.33%  (p=0.001 n=9+10)
String-8             3.56µs ± 4%    3.42µs ± 7%   -4.00%  (p=0.003 n=10+10)

name               old alloc/op   new alloc/op   delta
QueryEscape/#00-8     16.0B ± 0%      8.0B ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#01-8      128B ± 0%       64B ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#02-8     64.0B ± 0%     32.0B ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#03-8      128B ± 0%       64B ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#04-8      832B ± 0%      832B ± 0%     ~     (all equal)
PathEscape/#00-8      32.0B ± 0%     16.0B ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#01-8       128B ± 0%       64B ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#02-8      64.0B ± 0%     32.0B ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#03-8       128B ± 0%       64B ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#04-8       704B ± 0%      704B ± 0%     ~     (all equal)
String-8             1.84kB ± 0%    1.66kB ± 0%   -9.57%  (p=0.000 n=10+10)

name               old allocs/op  new allocs/op  delta
QueryEscape/#00-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#01-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#02-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#03-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
QueryEscape/#04-8      2.00 ± 0%      2.00 ± 0%     ~     (all equal)
PathEscape/#00-8       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#01-8       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#02-8       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#03-8       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
PathEscape/#04-8       2.00 ± 0%      2.00 ± 0%     ~     (all equal)
String-8               69.0 ± 0%      61.0 ± 0%  -11.59%  (p=0.000 n=10+10)

Updates #17860

Change-Id: I45c5e9d40b242f874c61f6ccc73bf94c494bb868
Reviewed-on: https://go-review.googlesource.com/134296
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>