Add skeleton copying collector

[fitzgen]

This commit adds the skeleton of a new copying collector, guarded behind a new "gc-copying" cargo feature. It compiles and implements the various Gc* traits to interface with the runtime and compiler, but is not yet actually functional.

The copying collector is intentionally excluded from the fuzzing config generator, as it isn't ready to be fuzzed yet.

cc #10329

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[alexcrichton]

The copying collector is intentionally excluded from the fuzzing config generator, as it isn't ready to be fuzzed yet.

Would it makes sense to leave comments as FIXMEs pointing to a tracking issue in a few locations (like fuzzing, the default set of features, etc) where this should go once it's ready?

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[fitzgen]

Would it makes sense to leave comments as FIXMEs pointing to a tracking issue in a few locations (like fuzzing, the default set of features, etc) where this should go once it's ready?

I have WIP fuzzing integration in some follow up work already so I don't think we need FIXMEs in the code at this time? But making a checklist somewhere of what we want done before enabling GC by default would be good to have. I'll whip that up in a little bit.