I noticed that Wasmtime was passing the spec tests despite having bits that were
known to be unimplemented. This should help the tests exercise those corners of
the spec.

Notably:

* Setting `i31ref` globals.

* Initializing tables and globals with `(ref.i31 (global.get $g))`.

* Table operations on `i31ref` tables.

* Accessing `anyref` globals and tables that are actually `i31ref`s. This is
  interesting to exercise outside of general subtyping because we have different
  paths in our inline GC barriers for `anyref`s that are actually `i31ref`s.