This implements the algorithm qsufsort using the sort package
as a sorting primitive. Its worst-case performance is O(N*log(N)), and it
uses only an additional slice of N ints of memory during creation.

Benchmarks (seconds):
           old    new
10k nulls          149    0.044
1M English corpus  32.0   3.6

R=gri, gri1
CC=golang-dev
https://golang.org/cl/3752044