Add support for using regalloc3 as the allocator back-end

[Amanieu]

Currently still a draft because:

• The regalloc3 crate isn't published yet.
• This is missing support for debug locations.

[cfallin]

After discussions with Cranelift maintainers, we've decided not to take this change: the main concern is maintenance burden, as the core Cranelift team is stretched relatively thin and we are in a place where we understand RA2 fully and can fix any bugs that arise or extend it as needed, but the same would not be true for a new allocator backend without a large investment of time. Thank you very much regardless for all of the discussions and best of luck with RA3!

[bjorn3]

Did the Cranelift meeting get moved earlier? According to the meeting invite it would be in half an hour from now.

[cfallin]

No, we're still meeting in a half-hour.

[cfallin]

(Sorry, my wording was ambiguous: I made the decision after discussing and bouncing ideas off a number of Cranelift maintainers. I'm happy to provide reasoning in the meeting soon.)