Move MIR towards a single kind of local
This PR modifies MIR to handle function arguments (`Arg`), user-defined variable bindings (`Var`), compiler-generated temporaries (`Tmp`), as well as the return value pointer equally. All of them are replaced with a single `Local` type, a few functions for iterating over different kinds of locals, and a way to get the kind of local we're dealing with (mainly used in the constant qualification/propagation passes).
~~I haven't managed to fix one remaining issue: A `StorageDead` not getting emitted for a variable (see the `TODO` in the test). If that's fixed, this is basically good to go.~~ Found the issue (an off-by-one error), fix incoming.
r? @eddyb for changes to constant qualification and propagation I'm not quite sure about
ff67da63ea