Previously this took 20 minutes, now it takes 2-4 seconds.

Changes:

 - Iterate over prefix length, from longest to shortest.
 - Build a map of prefixes and reuse it to avoid one loop.
 - When removing simple substrings, sort by length and only consider strings
   long enough to be worth checking. Saves 600ms out of 800ms.
 - Removed the option to generate uncrushed table.
 - Fix an unhandled error in n.walk(w, assignIndexes)

Change-Id: I321d2c2bd18f4918479500f3c61d5e59ee173f3d
Reviewed-on: https://go-review.googlesource.com/20029Reviewed-by: Nigel Tao <nigeltao@golang.org>