instead of variables. Several reasons:
- Encourage users of the API to minimize the number of creations and reuse Collate objects.
- Don't rule out the possibility of using initialization code for collators. For some locales
  it will be possible to have very compact representations that can be quickly expanded
  into a proper table on demand.
Other changes:
- Change name of root* vars to main*, as the tables are shared between locales.
- Added Locales() method to get a list of supported locales.

R=r
CC=golang-dev
https://golang.org/cl/6498107