strings: simplify Join using Builder

strings: simplify Join using Builder

The existing implementation has a bunch of special cases and
suffers an additional allocation for longer arrays. We can replace
this code with a simple implementation using Builder, improve
performance and reduce complexity.

name      old time/op    new time/op    delta
Join/0-8    3.53ns ± 3%    3.72ns ± 2%   +5.56%  (p=0.000 n=10+10)
Join/1-8    3.94ns ± 4%    3.40ns ± 4%  -13.57%  (p=0.000 n=10+10)
Join/2-8    57.0ns ± 3%    51.0ns ± 1%  -10.48%  (p=0.000 n=10+9)
Join/3-8    74.9ns ± 2%    65.5ns ± 4%  -12.60%  (p=0.000 n=10+10)
Join/4-8     105ns ± 0%      79ns ± 4%  -24.63%  (p=0.000 n=6+10)
Join/5-8     116ns ± 2%      91ns ± 4%  -21.95%  (p=0.000 n=10+10)
Join/6-8     131ns ± 1%     104ns ± 1%  -20.66%  (p=0.000 n=10+10)
Join/7-8     141ns ± 0%     114ns ± 4%  -18.82%  (p=0.000 n=9+10)

name      old alloc/op   new alloc/op   delta
Join/0-8     0.00B          0.00B          ~     (all equal)
Join/1-8     0.00B          0.00B          ~     (all equal)
Join/2-8     16.0B ± 0%     16.0B ± 0%     ~     (all equal)
Join/3-8     32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Join/4-8     96.0B ± 0%     48.0B ± 0%  -50.00%  (p=0.000 n=10+10)
Join/5-8     96.0B ± 0%     48.0B ± 0%  -50.00%  (p=0.000 n=10+10)
Join/6-8      128B ± 0%       64B ± 0%  -50.00%  (p=0.000 n=10+10)
Join/7-8      128B ± 0%       64B ± 0%  -50.00%  (p=0.000 n=10+10)

name      old allocs/op  new allocs/op  delta
Join/0-8      0.00           0.00          ~     (all equal)
Join/1-8      0.00           0.00          ~     (all equal)
Join/2-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
Join/3-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
Join/4-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
Join/5-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
Join/6-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)
Join/7-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)

Change-Id: I866a50e809c398512cb87648c955eaa4bf4d8606
Reviewed-on: https://go-review.googlesource.com/132895
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>