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.
Again, this is a matter of consistency. We already use & checks *everywhere* else and we're not going to change that. The use of `hasFlag` is an unnecessary inconsistency.