Make uPred itself independent of BI interface and just prove primitive laws (like in master branch)

See merge request FP/iris-coq!148

The BI interface is then instantiated using these laws.  In particular, this
shows that the rules we claim to be admissible actually are.

Fixes #193

Show that atomic_step is an AccElim so we can open invariants; get entirely rid of the Em ⊆ Eo side-condition

New atomic updates: defined as a fixed point with existential quantifier; intro lemma using class of Laterable assertions

If the BI is not affine, this should not happen, as it may lead to
information loss.

This commit fixes issue #190.

Thanks to @dfrumin.