Opportunity to further simplify internal cast representation

Right now almost all casts are represented as RefCast expressions, but ref.as_non_null is instead represented as a RefAs expression. This is a little awkward because the other two operations represented by RefAs are actually not casts at all, but rather conversions to and from externref. Having casts spread across two expression classes and having both cast and conversion operations represented by RefAs are sources of unnecessary complexity.

We could instead represent ref.as_non_null as a RefCast where the cast target heap type is a subtype of the input heap type. Another way to say this is that we could remove the RefAs representation of ref.as_non_null and instead opportunistically emit ref.as_non_null instead of ref.cast when the types work out.

Start by tracing how RefCast and RefAs expressions represent casts and conversions in Binaryen. Determine where ref.as_non_null is created, transformed, and emitted, then verify that the representation is unified without changing the intended ref.as_non_null versus ref.cast output behavior.