maybeCallVisit

Helper to dispatch the node if the method exist otherwise to a fallback.

This implementation with a fallback enables grouping of nodes. The user can either choose to implement a visit method for the specific node or a visit that receive the more generic node.

Example with a callback:

static void fallback(T)(ref Self s, ref Vis v, T node) {}
maybeCallVisit(this, visitor, node, &fallback!SpecNode);