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.