Initial nominal typing support (#3919)

In nominal mode, HeapType constructors besides the Signature constructor always
produce fresh types distinct from any previously created types. The HeapType
constructor that takes a Signature maintains its previous behavior of
constructing a canonical representative of the given signature because it is
used frequently throughout the code base and never in a situation that would
benefit from creating a fresh type. It is left as future work to clean up this
discrepancy between the Signature HeapType constructor and other HeapType
constructors.

TypeBuilder skips shape and global canonicalization in nominal mode and always
creates a fresh type for each of its entries. For this to work without any
canonicalization, the TypeBuilder allocates temporary types on the global Type
store and does not support building basic HeapTypes in nominal mode.

The new mode is not available in any of the Binaryen tools yet because it is
still missing critical functionality like the ability to declare subtyping
relations and correctly calculate LUBs. This functionality will be implemented
in future PRs.

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