compare incrementally. Also modified collation API to be more high-level
by removing the need for an explicit buffer to be passed as an argument.
This considerably speeds up Compare and CompareString.  This change also eliminates
the need to reinitialize the normalization buffer for each use of an iter. This
also significantly improves performance for Key and KeyString.

R=r, rsc
CC=golang-dev
https://golang.org/cl/6842050