[NFC] Simplify the HeapTypeStore (#5765)

Now that we support only isorecursive typing, which canonicalizes heap types by
the rec group rather than individually, we no longer need a canonicalizing store
for heap types. Simplify the `Store` implementation, which was previously
generic over both `HeapType`s and `Type`s, to instead work only for `Type`s.
Replace `globalHeapTypeStore` with a simple vector to keep canonicalized
`HeapTypeInfo`s alive. Also simplify global canonicalization to not replace heap
type uses, since that is already done separately as part of isorecursive rec
group canonicalization.

This simplification does require adding new information to
`CanonicalizationState`, but further work will simplify that away as well.

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