Implement Proposal: Reference Types

[ErikMcClure]

inNative must implement Reference Types, as they are a prerequisite to #41 interface types. This is made more complex because inNative already has cref types that behave in a similar way, except they are automatically converted. An intrinsic that can be used when sandboxed should be added that allows any integer to be converted to an anyref type, which behaves the same way a cref conversion does.