Alternative regalloc backends

The regalloc2 crate provides a good API (`VReg`, `PReg`, `Operand`, `Function`, etc) for interacting with a register allocator. It would be nice if other register allocator backends could be made available with the same API to simplify integration on the embedder side.

What I specifically have in mind is a "fast" backend that simply allocates registers in a single pass with the goal of minimizing compilation time. This will have a huge impact on compilation time since my compiler spends 80% of its time in register allocation. In practice I expect to use this for a tiering JIT where hot code can later be re-compiled using the slow regalloc backend that produces better code.