Code generation for Vigil

[lafrancef]

Entails deciding on the representation for virtual registers, and using that in our generated instructions.

Virtual registers will need at least an access kind (float, int, etc.), a force-to-hardware (to respect calling conventions; i.e. any return value will have to be sent to rax) and perhaps some kind of hint (we'd usually like to store things in rax to use the short forms of instructions).