Add separate GC heap tunables

[fitzgen]

Before this commit, the GC heap used a memory with identical tunables as regular Wasm linear memories.

After this commit, the GC heap now uses a memory with its own tunables (gc_heap_reservation, gc_heap_guard_size, gc_heap_reservation_for_growth, and gc_heap_may_move) so that GC heaps can be configured independently from linear memories. These new GC heap tunables are also exposed as CLI flags and Config methods. We also generate pseudorandom values for these tunables in our fuzzing config generator.

The pooling allocator uses the same pool for memories and GC heaps, so we validate that if you are using GC and the pooling allocator, then the new tunables must match the memory tunables.

cc #11256

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[fitzgen]

@alexcrichton okay I created a MemoryTunables type that brings together a enum MemoryKind { LinearMemory, GcHeap } and a &Tunables and then exposes methods to get either self.tunables.gc_heap_* or self.tunables.memory_* depending on the MemoryKind. Then I plumbed this through everywhere we were using Tunables in memory-related code. The hope is that this should make it so that we can't ever access the wrong tunable, and have completely unified handling/code paths with no duplication between GC heap vs linear memory.

Mind taking another look?

[fitzgen]

(Probably easiest to just view the diff against main, rather than on a per-commit basis)

[alexcrichton]

👍