exp/locale/collate: preparation for adding Search API. Also changed the collate API
further to how (I believe) it will end up being.
It is nicer to separate search from sorting functionality. Collation needs tables that
are not needed by search and vice-versa.  The common functionality is separated out
in the Weigher interface.  As this interface is very low-level, it will be moved to
a sub package (colltab) in a next CL.
The types that will move to this package are Weigher, Elem, and Level.  The addition
of Elem allows for removing some of the duplicate code between collate and collate/build.
This CL also introduces some stubs for a higher-level API for options. The default
proposed options are quite complex and require the user to have a decent understanding
of Unicode collation.  The new options hide a lot of the complexity.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7058051
exp/locale/collate: added functionality for sorting.
Eliminates the need for the user to fiddle with keys.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7060051