Enforce index-type-size limits at AOT generation (i.e. "compile") time.

[ltratt]

In the JIT IR we have various type size limits (e.g. 24 bit indexes) that we have to check dynamically, even though most of them can easily be enforced at AOT generation (i.e. "compile") time. For example, 9c91e81 shows that we can check that the type indexes we generate at AOT always fit within 24 bits: that would make the code in "main" yk clearer, because we'd know that any unwraps were guaranteed not to fail (unless we've done something stupid; i.e. the unwrap would become equivalent to an assert rather than the current "we've not worked out how to handle this case yet").