Add `Atomic` instances for all atomic `heap_lang` constructs.

See merge request iris/iris!317

Also removed some admissible instances:

- `Atomic s (ResolveProph (Val v1) (Val v2))` (this one was already admissible)
- `Atomic s Skip` (became admissible due to the instance for β)

Prove that non-value WP is contractive.

See merge request !316

Let `iProp` refer to `uPred ... : Type` instead of `uPredO ... : ofeT`

See merge request iris/iris!314

Reorder Requires so that we do not depend of Export bugs.

See merge request iris/iris!313

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 (??).

Add `iStopProof` tactic

Closes #265

See merge request !311

This commit closes issue #265.

We have these instances for all other logical operations too to support
setoid rewriting in both directions.

fix typo in the docs

See merge request iris/iris!310