net/url: check for lowercase before uppercase in shouldEscape

net/url: check for lowercase before uppercase in shouldEscape

Most chars in URLs are lowercase, so check that first.

Performance change:

String-4               7.62µs ± 1%    7.27µs ± 3%  -4.61%  (p=0.008 n=5+5)
QueryEscape/#00-4      92.6ns ± 3%    90.3ns ± 1%  -2.48%  (p=0.016 n=5+5)
QueryEscape/#01-4       515ns ± 4%     510ns ± 2%    ~     (p=0.683 n=5+5)
QueryEscape/#02-4       375ns ± 1%     343ns ± 1%  -8.52%  (p=0.008 n=5+5)
QueryEscape/#03-4       758ns ± 1%     699ns ± 1%  -7.83%  (p=0.008 n=5+5)
QueryEscape/#04-4      6.06µs ± 1%    5.74µs ± 1%  -5.38%  (p=0.008 n=5+5)
PathEscape/#00-4        140ns ± 1%     135ns ± 2%  -3.85%  (p=0.008 n=5+5)
PathEscape/#01-4        511ns ± 3%     507ns ± 3%    ~     (p=0.587 n=5+5)
PathEscape/#02-4        372ns ± 1%     342ns ± 2%  -8.22%  (p=0.008 n=5+5)
PathEscape/#03-4        747ns ± 1%     685ns ± 1%  -8.30%  (p=0.008 n=5+5)
PathEscape/#04-4       5.94µs ± 1%    5.64µs ± 3%  -4.98%  (p=0.008 n=5+5)
QueryUnescape/#00-4     111ns ± 4%     110ns ± 2%    ~     (p=0.952 n=5+5)
QueryUnescape/#01-4     390ns ± 0%     391ns ± 2%    ~     (p=0.714 n=5+5)
QueryUnescape/#02-4     297ns ± 5%     295ns ± 3%    ~     (p=0.524 n=5+5)
QueryUnescape/#03-4     543ns ± 3%     556ns ± 2%  +2.39%  (p=0.032 n=5+5)
QueryUnescape/#04-4    3.23µs ± 3%    3.22µs ± 2%    ~     (p=1.000 n=5+5)
PathUnescape/#00-4      111ns ± 1%     110ns ± 3%    ~     (p=0.881 n=5+5)
PathUnescape/#01-4      389ns ± 2%     386ns ± 2%    ~     (p=0.444 n=5+5)
PathUnescape/#02-4      297ns ± 1%     295ns ± 3%    ~     (p=0.738 n=5+5)
PathUnescape/#03-4      557ns ± 3%     553ns ± 2%    ~     (p=0.810 n=5+5)
PathUnescape/#04-4     2.94µs ± 2%    2.97µs ± 2%    ~     (p=0.222 n=5+5)

Change-Id: I7e6d64cd5f8f5218cb40f52f0015168a8674aabb
Reviewed-on: https://go-review.googlesource.com/c/go/+/168883
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>