exp/locale/collate: implementation of tailorings and table generation.

exp/locale/collate: implementation of tailorings and table generation.
Tailorings are represented by removing and reinserting entries from a linked list.
After all tailorings are done, missing weights are computed and verified.
This implementation assumes that entries that are used in expansions are not
reinserted at a later point.  This considerably simplifies the implementation.

R=r
CC=golang-dev
https://golang.org/cl/6739052