Cranelift: add option to use new single-pass register allocator.

[cfallin]

In bytecodealliance/regalloc2#181, @d-sonuga added a fast single-pass algorithm option to regalloc2, in addition to its existing backtracking allocator. This produces code much more quickly, at the expense of code quality. Sometimes this tradeoff is desirable (e.g. when performing a debug build in a fast-iteration development situation, or in an initial JIT tier).

This PR adds a Cranelift option to select the RA2 algorithm, plumbs it through to a Wasmtime option, and adds the option to Wasmtime fuzzing as well.

An initial compile-time measurement in Wasmtime: spidermonkey.wasm builds in 1.383s with backtracking (existing algorithm), and 1.065s with single-pass. The resulting binary runs a simple Fibonacci benchmark in 2.060s with backtracking vs. 3.455s with single-pass.

Hence, the single-pass algorithm yields a 23% compile-time reduction, at the cost of a 67% runtime increase.

Fixes #9596.

[Amanieu]

Should this selection be automatic with opt_level=none?

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[bjorn3]

In that case we should probably get an opt level between none and speed which uses the better regalloc but keeps egraph optimizations disabled given that the better regalloc has a significantly higher improvement to runtime performance than egraph optimizations.

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[cfallin]

For now at least, I think I'd prefer to keep it an opt-in default -- let's let it bake in wasmtime's continuous fuzzing for a little longer. We can always switch the default later.

@alexcrichton updated to add cargo-vet, could you rubber-stamp the new commit? Also fixed silly issues in fuzz build (which I never test beforehand because Ocaml; I should fix my setup!).