Expose atomic lifetimes opaquely in the API

This extends the lifetime signature to expose the notion of atomic
lifetimes and how they can embed into normal lifetimes. This allows for
a future lemma where I will state that a particular new lifetime is
atomic, thus allowing for the application of the lifetime-ending lemma
at a later point.

This has been done opaquely rather than exposing directly the definition
of `atomic_lft` to try to keep the API clean.