Remove dependency on desugarer from core/ForkMergeParser.java

[paulgazz]

Currently, the parser has a dependency on superc.cdesugarer.CContext for some static state in that class. The core/ directory is grammar-independent and the grammars depend on it creating a circular dependency. Using static state also breaks idempotency of the desugarer.

This can be improved by collecting parser error conditions as an instance field on the parser itself (when enabled by a set...() method, e.g., setNewErrorHandling()), and having the desugarer query the parser instance's state instead.