make atomic_acc typeclasses-opaque

Use telescopes for atomic accessors, updates and triples; improve mask handling; add notation for all of them

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

* move PROP-envs definitions to environments.v so that we can control them without pulling in coq_tactics
* use reduction-controlled `pm_default` for proofmode accessors

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