Adds desugar indirection

h_ch's return type is uint, not bytes; fixed validate_test_ab accordingly, even though it worked before with the wrong union member name (probably because token is the first thing in the struct). Oh C, stay classy.

Desugaring functions are all done! But we're not ready for the CF backends yet because desugared structures still need to be stashed in the parser.

Expanded vtable for methods to determine whether a combinator is ok to use with regular or CF grammars.

Benchmark is actually broken right now, for mysterious double-free related reasons. I will dig into this tomorrow.