Builtin::call()

Integration between layer1 and the raw C++ is complete, but as yet untested.

builtin->call(nargs, ...) should call the builtin with the nominated args. There's a Slot occ wrapper to automatically count args for you, so it's normally slot->call(...)

I'll test it, in situ, when I'm feeling braver, but it works in vitro (in my test/ setup, outside coldstore.)

We'll have to work on a (de)mangler to make it really useful.

There's support for stack->Tuple and Tuple->stack conversion, too.