Use custom handle type to joints rather than unique_ptr

[sansumbrella]

The underlying joint has ambiguous ownership. If either of the b2Bodies it connects are destroyed, the joint will be destroyed by Box2d. This leads to double-deletion if either body is destroyed before the unique_ptr returned by our wrapper.

So we need a handle that can remove the joint, but will (hopefully) not delete it if the joint has already been destroyed. Maybe it keeps both bodies alive by containing a shared_ptr to those involved in the joint as well as the non-owning raw pointer to the joint.

[sansumbrella]

Handle should implement joint destruction listener. Don't scope the lifetime of the joint to the basic handle. Instead, have an optional scoped version that destroys the joint.