Implemented the TaggedGrammar.toGrammar function.
Added an additional parameter to IBackend.Invoke so the TaggedGrammar can be passed into the backend.
Cleaned up the Compiler.compile function -- it now uses the 'choice' workflow, and writes informational messages to the logger.
The fsyacc backend has been temporarily disabled -- it needs a few more TaggedGrammar-related modifications before it'll compile again.