Also, add a benchmark variant ("SkipServe") that only benchmarks the
ServeMux handler selection path.

name                  old time/op    new time/op    delta
ServeMux_SkipServe-4    74.2µs ± 2%    60.6µs ± 1%   -18.31%   (p=0.000 n=10+9)

name                  old alloc/op   new alloc/op   delta
ServeMux_SkipServe-4    2.62kB ± 0%   0.00kB ±NaN%  -100.00%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
ServeMux_SkipServe-4       180 ± 0%        0 ±NaN%  -100.00%  (p=0.000 n=10+10)

Updates #25383

Change-Id: Icfbb3b977e309093d032e922d1b4f254df6f5955
Reviewed-on: https://go-review.googlesource.com/116378
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>