- change way of protection from O(N^2) on duplicate values.
  Previous algorithm does additional comparisons and swaps
  on every split pass.
  Changed algorithm does one ordinal quicksort split pass,
  and if distribution is skewed, then additional pass to
  separate pivot's duplicates.
  Changed algorithm could be slower on very ununique slice,
  but it is still protected from O(N^2).

- increase small slice size and do simple shell sort pass
  to amortize worst case on small slices.
  Small slice has higher probability to have skewed
  distribution, so lets sort it with simpler algorithm.

benchmark                 old ns/op      new ns/op      delta
BenchmarkSortString1K     458374         388641         -15.21%
BenchmarkSortInt1K        217851         181796         -16.55%
BenchmarkSortInt64K       20539264       16730340       -18.54%
BenchmarkSort1e2          98668          95554          -3.16%
BenchmarkSort1e4          20278500       18316829       -9.67%
BenchmarkSort1e6          3215724392     2795999911     -13.05%

number of operations:
       Size:         Total:     Swap:     Less:
                          %         %         %
Sort     100  Avg    -5.98%   -18.43%    -1.90%
Sort     100  Max   -14.43%   -16.02%    -4.51%
Sort     300  Avg    -7.50%   -12.76%    -5.96%
Sort     300  Max   -11.29%    -9.60%    -4.30%
Sort    1000  Avg   -12.13%   -11.65%   -12.25%
Sort    1000  Max   -13.81%   -11.77%   -11.89%
Sort    3000  Avg   -14.61%    -9.30%   -15.86%
Sort    3000  Max   -15.81%    -8.66%   -15.19%
Sort   10000  Avg   -16.10%    -8.47%   -17.80%
Sort   10000  Max   -17.13%    -7.63%   -16.97%
Sort   30000  Avg   -17.46%    -7.56%   -19.57%
Sort   30000  Max   -18.24%    -7.62%   -17.68%
Sort  100000  Avg   -18.83%    -6.64%   -21.33%
Sort  100000  Max   -19.72%    -6.70%   -20.96%
Sort  300000  Avg   -19.61%    -6.16%   -22.30%
Sort  300000  Max   -20.69%    -6.15%   -21.81%
Sort 1000000  Avg   -20.42%    -5.58%   -23.31%
Sort 1000000  Max   -21.54%    -5.56%   -23.61%

Change-Id: I23868e8b52b5841b358cd5403967c9a97871e4d5
Reviewed-on: https://go-review.googlesource.com/15688Reviewed-by: Russ Cox <rsc@golang.org>