Adding a hint without a database now triggers a deprecation warning in
Coq master (https://github.com/coq/coq/pull/8987).

make pm_maybe_wand a BI connective; reduce BI connectives and option combinators in the proofmode with cbn

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

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

This follows the proof at https://en.wikipedia.org/wiki/L%C3%B6b's_theorem#Proof_of_L%C3%B6b's_theorem

New IntoAcc typeclass to decouple creating and elliminating accessors; ElimInv supports both with and without Hclose

Fixes #156

Fixes #177

The old one is admissable.

Thanks to @jtassaro and @jung.

Based on an earlier MR by @jung.

As suggested by @jjourdan, and proved in the ordered RA model by @amintimany.

This should solve the paradox in #149.