Because we smuggle them through we need to make sure they're not
holding references that will go invalid- we can make no promises about
how long they stay alive. Although maybe bounding to context lifetime
is OK? But anyway.
It turns out that rust can't really reason about the relationship
between the runtime lifetime and the context lifetime in a way that is
actually usable. This removes the lifetime stuff in favor of reference
counting the runtime itself, via a block that we embed in the
pointer. This, I think, it the least worst option here.
