net/url: Reduce allocation on parsing URL func

Build result string via string.Builder to avoid allocation.
As side effect some performance boots.

name                 old time/op    new time/op    delta
QueryUnescape/#00-4     114ns ± 0%      98ns ± 1%  -13.89%  (p=0.000 n=4+5)
QueryUnescape/#01-4     401ns ± 2%     383ns ± 1%   -4.54%  (p=0.008 n=5+5)
QueryUnescape/#02-4     300ns ± 2%     274ns ± 2%   -8.66%  (p=0.008 n=5+5)
QueryUnescape/#03-4     564ns ± 2%     542ns ± 2%   -4.04%  (p=0.008 n=5+5)
QueryUnescape/#04-4    3.27µs ± 2%    3.34µs ± 8%     ~     (p=0.690 n=5+5)
PathUnescape/#00-4      112ns ± 2%      99ns ± 3%  -11.25%  (p=0.008 n=5+5)
PathUnescape/#01-4      392ns ± 2%     374ns ± 6%     ~     (p=0.063 n=5+5)
PathUnescape/#02-4      296ns ± 2%     274ns ± 2%   -7.43%  (p=0.008 n=5+5)
PathUnescape/#03-4      556ns ± 2%     537ns ± 1%   -3.45%  (p=0.008 n=5+5)
PathUnescape/#04-4     2.99µs ± 1%    3.00µs ± 1%     ~     (p=0.690 n=5+5)

name                 old allocs/op  new allocs/op  delta
QueryUnescape/#00-4      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
QueryUnescape/#01-4      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
QueryUnescape/#02-4      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
QueryUnescape/#03-4      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
QueryUnescape/#04-4      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
PathUnescape/#00-4       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
PathUnescape/#01-4       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
PathUnescape/#02-4       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
PathUnescape/#03-4       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)
PathUnescape/#04-4       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.008 n=5+5)

Change-Id: I7cba5eb53bebef7b1fdd44598eed47241ce83167
Reviewed-on: https://go-review.googlesource.com/c/go/+/166463


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>