This is simpler because we don't "discover" functions to compile as we
go, we just compile all the ones we can find, and functions have
pre-defined exports. This is good and useful to us as we can now refer
to functions in different modules by known indices, but it *does* make
me wonder what we're going to do for compiling generic specializations.
The previous technique was better for that sort of thing.
This is all predicated on the idea that I want to have
partially-compiled modules, which I can't really say why I want it. If
I'm happy to just compile things cross module in the same kind of
space then it's much easier to go back to the function key way of
working.
