Add support for incremental parsing.

Right now the entrypoint just causes a full parse to happen.  But the LS code is
cleaned up to take advantage of it appropriately.

This will not be checked in until all the incremental parsing work is complete.
Got it. But why does VariableStatement need an extra level of indirection? Why not VariableStatement *is-a* VariableDeclarationList instead of *has-a*.