Make `compose` a notation rather than a definition

This makes life a lot easier; we don't have to be constantly unfolding it, and we can avoid `Local Arguments compose /.` and `simpl rewrite` in lots of places.
I'm not terribly surprised.  I suspect more things are `simpl`ed, so that it's harder to pick out the right functions.  In any case, I'm fine relying on `apply` for now.