This is a replacement for pending CL 2219042. It only contains
the raw suffixarray functionality with two methods:

- New       create a new index from some data
- Lookup    lookup occurences of a bytes slice in the data

Any other functionality (dealing with multiple data sets and
the corresponding position lists) is generic and doesn't have
to be part of this package.

Known performance bug: This implementation works fine for data sets
up to several megabytes as long as it doesn't contain very long
contiguous sequences of equal bytes. For instance, index creation for
all .go files under GOROOT (250KLOCs, approx. 9MB) takes ~50s on
2.66 GHz Intel Xeon as long as test/fixedbugs/257.go is excluded.
With that file, index creation times takes several days. 257.go contains
a string of 1M smiley faces.

There are more sophisticated suffixarray creation algorithms which
can handle very long common prefixes. The implementation can be
updated w/o the need to change the interface.

R=rsc, r, PeterGo
CC=golang-dev
https://golang.org/cl/2265041