Prior to this change, the interpreter would use an incorrect `FuncRef` for accessing functions from the function store. This is now clarified and fixed by a new type--`FuncIndex`.
* Rewrite interpreter generically
This change re-implements the Cranelift interpreter to use generic values; this makes it possible to do abstract interpretation of Cranelift instructions. In doing so, the interpretation state is extracted from the `Interpreter` structure and is accessed via a `State` trait; this makes it possible to not only more clearly observe the interpreter's state but also to interpret using a dummy state (e.g. `ImmutableRegisterState`). This addition made it possible to implement more of the Cranelift instructions (~70%, ignoring the x86-specific instructions).
* Replace macros with closures
This is an incomplete version of a Cranelift IR interpreter: only a small subset of instructions are implemented and (known) missing parts are marked with TODO or FIXME.
