prototype: enforce single-realm invariants for roots and weak pointers

[shruti2522]

As mentioned by @jedel1043 in #68 :

We might have some ways to statically ensure Root is not used with a different realm, like relating them to the GcContext itself by some invariant lifetime, but that's a future improvement that we can do while iterating on this

updates the API to use compile time lifetime checks ('id) to ensure
that GC pointers, Root and WeakGc can never cross into a different GcContext

[nekevss]

LGTM