TypeBuilder (#3418)

Introduce TypeBuilder, a utility for constructing heap types in terms of other
heap types that may have not yet been defined. Internally, it works by creating
HeapTypes backed by mutable HeapTypeInfos owned by the TypeBuilder. That lets
the TypeBuilder create temporary Types that can refer to the TypeBuilder-managed
HeapTypes. Those temporary Types can in turn be used to initialize the very
HeapTypes they refer to. Since the TypeBuilder-managed HeapTypes are only valid
for the lifetime of their TypeBuilder, there is a canonicalization step that
converts them into globally interned canonical HeapTypes.

This PR allows HeapTypes to be built in terms of as of yet undefined HeapTypes,
but it currently errors out in the presence of recursive types. Supporting
recursive types will require further work to canonicalize them into finite,
acyclic representations. Currently any attempt to compare, print, or otherwise
manipulate recursive types would infinitely recurse.

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