Add an associated type bound to enforce the rule that `InHeap` and `IntoHeap` types must be paired 1-1. (`IntoHeap::In` already has the corresponding bound in the opposite direction. I think both bounds are necessary to enforce the rule, though honestly it is not the sort of rule that anyone is likely to break.)
Add an associated type bound to enforce the rule that `InHeap` and `IntoHeap` types must be paired 1-1. (`IntoHeap::In` already has the corresponding bound in the opposite direction. I think both bounds are necessary to enforce the rule, though honestly it is not the sort of rule that anyone is likely to break.)

jorendorff/cell-gc,jorendorff/cell-gc