Lang lemmas

See merge request iris/iris!324

There is no need to include the `(∃ P', □ ▷ (P  P')  ...` since we
get closure under `▷ □ ` from regular invariants.

Added a stronger version of cinv_open_strong

See merge request iris/iris!326

Simplify definition of invariant model.

See merge request iris/iris!327

Due to the new semantic invariants (!319) we no longer need to
close the model (i.e. `inv_def`) to be contractive, the semantic
invariant definition (i.e. `inv`) is already contractive.

Update gitignore for compatibility with Coq master

See merge request iris/iris!325

See https://github.com/coq/coq/pull/10947 (.coqdeps.d now uses the name
of the Coq Makefile) and https://github.com/coq/coq/pull/8642 (Coq now
generates empty interface files *.vos when compiling).

move docs around

See merge request iris/iris!323

make gFunctors_lookup a local coercion to avoid ambiguous paths

See merge request iris/iris!322

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)