The trie code looks a lot like the trie in exp/norm. It uses different
types, however.  Also, there is only a lookup for []byte and the unsafe
lookup methods have been dropped, as well as sparse mode.
There is now a method for generating a trie. To output Go code, one now needs
to first generate a trie and then call print() on it.

R=r, r, mpvl
CC=golang-dev
https://golang.org/cl/5966064