Unify reference types

[dcodeIO]

This change unifies the so far available reference types by rebasing them on the non-nullable variants (matching the other types, e.g. string is non-nullable) while refactoring the canonical names like externref to become aliases. Hence all base types are non-nullable now, with the reference types prefixed with ref_ to avoid naming collisions (like string != ref_string) for the time being. The canonical names as per the spec remain as aliases of the nullable types. Using externref as an example:

AS type	Wasm type	Canonical Wasm type & AS alias
ref_extern	(ref extern)	-
ref_extern | null	(ref null extern)	externref

Idea is that we won't have to write nonnull<stringref> etc., but can reuse TS's | null notation in the transition to reference types, eventually leading full circle to writing string | null again, since once the respective naming collisions like string != ref_string can be resolved, we might simply drop the ref_ prefix.

Given that the previous type names remain as aliases and most of this is experimental anyway, this should otherwise be a non-functional change.

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