Cranelift: Implement the lowering ISLE context with `LowerCtx` directly

Right now, it is easy to call Rust from ISLE, but calling back into ISLE from Rust (e.g. to take advantage of optimized sign/zero extend functionality that is in ISLE) is difficult because you don't usually have the whole `IsleContext` anymore.

But you usually have a `LowerCtx`.

And conceptually they are the same: context used when lowering.

Would be nice to combine them into one thing and implement the ISLE lowering context trait with `LowerCtx` directly.