[fuzz] Configure the differential target

[abrown]

This change is a follow-on from #4515 to add the ability to configure
the differential fuzz target by limiting which engines and modules are
used for fuzzing. This is incredibly useful when troubleshooting, e.g.,
when an engine is more prone to failure, we can target that engine
exclusively. The effect of this configuration is visible in the
statistics now printed out from #4739.

Engines are configured using the ALLOWED_ENGINES environment variable.
We can either subtract from the set of allowed engines (e.g.,
ALLOWED_ENGINES=-v8) or build up a set of allowed engines (e.g.,
ALLOWED_ENGINES=wasmi,spec), but not both at the same time.
ALLOWED_ENGINES only configures the left-hand side engine; the
right-hand side is always Wasmtime. When omitted, ALLOWED_ENGINES
defaults to [wasmtime, wasmi, spec, v8].

The generated WebAssembly modules are configured using
ALLOWED_MODULES. This environment variables works the same as above
but the available options are: [wasm-smith, single-inst].

[abrown]

Here is an example from running the target locally:

$ ALLOWED_MODULES=-wasm-smith cargo +nightly fuzz run differentia
...
=== Execution rate (384 successes / 2000 attempted modules): 19.20% ===
        wasmi: 0.96%, spec: 8.85%, wasmtime: 83.97%, v8: 6.22%
        wasm-smith: 0.00%, single-inst: 100.00%

[abrown]

@jameysharp, you've had some opinions on this in the past so I tagged you for review (?): I really would have preferred to use &[&str] everywhere (in the static variables, function parameters, etc.) but once env gave me back Strings this all started sliding toward Vec<String>. This makes for some awkward match and if statements. Do you see an easy way to convert to slices and &str or even just one of those?

[jameysharp]

It's true: I have, at times, had opinions! 😆

I don't feel very strongly about any of these review comments. This PR is fine as-is, so I'm marking this approved. But if you want to take a little more time on it I think these suggestions are improvements.

[github-actions]

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[jameysharp]

I think you've addressed all my suggestions. Thanks! I hope you found these suggestions helpful.

[abrown]

I think you've addressed all my suggestions. Thanks! I hope you found these suggestions helpful.

Yeah, they were great!