sort: implement stable sorting

This CL provides stable in-place sorting by use of
bottom up merge sort with in-place merging done by
the SymMerge algorithm from P.-S. Kim and A. Kutzner.

The additional space needed for stable sorting (in the form of
stack space) is logarithmic in the inputs size n.
Number of calls to Less and Swap grow like O(n * log n) and
O(n * log n * log n):
Stable sorting random data uses significantly more calls
to Swap than the unstable quicksort implementation (5 times more
on n=100, 10 times more on n=1e4 and 23 times more on n=1e8).
The number of calls to Less is practically the same for Sort and
Stable.

Stable sorting 1 million random integers takes 5 times longer
than using Sort.

BenchmarkSortString1K      50000       328662 ns/op
BenchmarkStableString1K    50000       380231 ns/op  1.15 slower
BenchmarkSortInt1K         50000       157336 ns/op
BenchmarkStableInt1K       50000       191167 ns/op  1.22 slower
BenchmarkSortInt64K         1000     14466297 ns/op
BenchmarkStableInt64K        500     16190266 ns/op  1.12 slower

BenchmarkSort1e2          200000        64923 ns/op
BenchmarkStable1e2         50000       167128 ns/op  2.57 slower
BenchmarkSort1e4            1000     14540613 ns/op
BenchmarkStable1e4           100     58117289 ns/op  4.00 slower
BenchmarkSort1e6               5   2429631508 ns/op
BenchmarkStable1e6             1  12077036952 ns/op  4.97 slower

R=golang-dev, bradfitz, iant, 0xjnml, rsc
CC=golang-dev
https://golang.org/cl/9612044