Add the "null" garbage collector

[fitzgen]

The null collector does not actually collect any garbage, it simply bump-allocates until the heap is exhausted, at which point further allocation attempts will fail. It does not require any GC barriers.

Users can configure which collector to use via the wasmtime::Config::collector method or the -C collector=drc|null CLI flag. The wasmtime::Collector enumeration, similar to the wasmtime::Strategy enumeration but for choosing a collector rather than a compiler, additionally has a table summarizing the properties and characteristics of our current set of collector implementations.

Finally, we also run .wast tests that exercise GC types under both the DRC and null collectors. I tried to avoid running tests that are not related to GC under both configurations just to avoid a combinatorial blow up.

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[alexcrichton]

Very nice! I like how this shaped up. I hope the #[cfg] wasn't too much to add, but it all looks relatively focused to be so it's hopefully just a one-time cost.

[fitzgen]

Very nice! I like how this shaped up.

Thanks!

I hope the #[cfg] wasn't too much to add, but it all looks relatively focused to be so it's hopefully just a one-time cost.

Indeed, it was mostly some shuffling to get rid of the disabled collector impl and then realizing that various things that were DRC-specific modules could actually be made a little more generic and reused with other collectors. I think adding the copying collector will be relatively small in comparison to this PR, which is kind of funny.