fluent 0.12.0

This release of fluent brings support for version 0.9 of the Fluent Syntax
spec. The FluentBundle API remains unchanged. Files written in valid Syntax
0.8 may parse differently in this release. See the compatibility note below.

• Implement Fluent Syntax 0.9.

  Most of the changes introduced in Syntax 0.9 affect the full tooling AST
  and thus do not apply to the fluent package which uses a different data
  structure for the result of the parsing, optimized for the runtime
  performance.

  Syntax features deprecated in Syntax 0.8 have been removed in Syntax 0.9.
  The support for them have been removed in this release of fluent too.

  Consult the full Syntax 0.9 changelog for details.

Backward-incompatible changes:

• VariantLists are no longer valid syntax.