Is there a way to make i31 shared?

[tlively]

As @rossberg pointed out in #55 (comment), it would be nice if we could allow i31ref in shared contexts. The trouble is that it would then have to be a subtype of both eqref and (shared eqref) (at least AFAICT), and we have never allowed a type to have multiple supertypes before, making this a risky new frontier. We should experiment with this and see whether we run into problems in practice.