Rewrite Spacebars optimizer to use Visitors
Comments
Move Spacebars compiler to SpacebarsCompiler

The pattern of one package defining Spacebars and another augmenting it with methods like Spacebars.parse is too confusing.

The symbols `parse`, `compile`, `TemplateTag` are now on `SpacebarsCompiler`.  You can now access `SpacebarsCompiler.optimize`, and we can start exposing more symbols.