for dealing with CLDR files:
- Add now taxes a list of indexes of colelems that are variables. Checking and
  handling is now done by the Builder.  VariableTop is now also properly generated
  using the Build method.
- Introduced separate Builder, called Tailoring, for creating tailorings of root
  table.  This clearly separates the functionality for building a table based on
  weights (the allkeys* files) versus tables based on LDML XML files.
- Tailorings are now added by two calls instead of one: SetAnchor and Insert.
  This more closely reflects the structure of LDML side and simplifies the
  implementation of both the client and library side.  It also preserves
  some information that is otherwise hard to recover for the Builder.
- Allow the LDML XML element extend to be passed to Insert.  This simplifies
  both client and library implementation.

R=r
CC=golang-dev
https://golang.org/cl/6454061