Visit callable objects

[jakobjpeters]

Closes #50.

[timholy]

Sorry I didn't see this earlier. It's a nice idea, but I do worry about the unrestricted signature for x: this replaces the default "do-nothing" fallback with a method that tries to do something.

I wonder if rather than relying on dispatching the callee, what if we put a check for !isempty(methods(x)) && _visit(...) in the caller of the method you've written?