The general idea is to first import/export modules which are further
than the current one, and then import/export modules which are close
dependencies.

This commit tries to use the same order of imports for every file, and
describes the convention in ProofGuide.md. There is one exception,
where we do not follow said convention: in program_logic/weakestpre.v,
using that order would break printing of texan triples (??).

All the `env` operations are prefixed `env_`, so this is more consistent.

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