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.
Me too.  You should possibly make a note to this effect in `STYLE.md` so that someone else doesn't come along and decide to do this, though.  The way it's currently worded seems a tiny bit deceptive.