Non-moving generational GC via sticky mark bits

[fitzgen]

Doesn't seem to actually be a win yet, but it adds nice infrastructure and should be a win with a little bit more work.

A few things we can do:

• Add a flag to PageHeader for if anything in the page has been pinned. This should take a sizable constant time chunk out of root collection.

• Another flag to PageHeader for if anything on the page has been marked. This way we can choose to only sweep whole pages of garbage at a time. This is independent of generational GC, but fits with its philosophy: focus work where its most likely to be effective, ignore other elsewhere.

• Revisit how incremental sweeping will work out with this new generational GCing.

• Reuse the store buffer for rust heap to gc heap edges (would require some way to remove remembered edges when roots are un-pinned). This will allow much faster root enumeration, but perhaps at the price of costlier individual allocations.

• Tweak the heuristics on when to do a generational GC vs full GC. Right now, it only happens when the store buffer is full (and I haven't even verified how often that happens, or if it is even happening at all). Also, how much more fuel a nursery collection should add to the gc_counter.

GENERATIONAL W/ STICKY MARK BITS ===============================================

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CONTROL ========================================================================

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