Validate global imports

[stevenfontanella]

• Add a RuntimeGlobal class to wrap the global's runtime value as well as its definition so we can check its type + mutability for import validation
  • Global imports are invariant for mutable globals and covariant for immutable globals:
    . Mutability must match as well.
• Fix old_import.wast to account for instantiations which should correctly fail. This test is now mostly the same as test/spec/imports.wast, but there are small differences so I plan to keep it until the upstream testsuite imports.wast passes.

Example error message:

[trap Imported global Mref_ex.g-var-func with type: (mut (ref func)) isn't compatible with import declaration: (import "Mref_ex" "g-var-func" (global $gimport$0 (mut funcref)))
]

Part of #8261.

[tlively]

Does this mean the test is wrong or that our implementation is wrong?

[stevenfontanella]

Our implementation is wrong, table types should be invariant for imports but I implemented them as covariant (same mistake as Java): link. I'll address this in the next PR.

[tlively]

LGTM!