Update version in URL for Iris appendix

See merge request iris/iris!321

I'd consider (in addition or alternative) to link to a new
http://plv.mpi-sws.org/iris/appendix-master.pdf, always matching the latest
version.

Soundness lemma for internal equality of `uPred`.

See merge request iris/iris!315

Semantic invariants

See merge request iris/iris!319

drop Coq 8.7 and add 8.10

Closes #242

See merge request iris/iris!320

rename LitErased -> LitPoison

See merge request iris/iris!318

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 iris/iris!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