feat: fun syntax in route

[chsukivra]

Not tested at all, but half-baked changes to be able to give xref enough information to detect if the routing table refers to exported functions, and that exported functions in controllers have references to them.

I'd like this to be to other way around, turn {Mod, Fun} tuples into type external fun Mod:Fun/A values in the routing lookup table. Instead because it was easy I am rewriting funs to {Mod, Fun} tuples.

I'm hearing that this is the direction you're already going.