defer looking up function calls until after parsing

[trillllian]

math_ast_function_call looks up the function name in its constructor. this has the unfortunate effect that user function definitions can't refer to userfuncs defined in the future (which is a regression relative to the old parser)

where do i defer this lookup to though??? redo it on every call to getlen/haslabel/evaluate? make it cache the result somewhere? (all the ast_node functions are marked const, but i guess i can delete the consts.)

probably fine to redo it, it's two hashmap lookups in total

[trillllian]

actually i think the only think which needs deferring is the "unknown function" error; whether it's a reference to a builtin or not is safe to determine at parsing time