The view camera.

Closes #257

See merge request iris/iris!516

Remove `pure_forall_2` from BI interface + BI notation for `¬`

See merge request iris/iris!526

Make adequacy apply to multiple initial threads

See merge request iris/iris!485

put it into a type class `BiPureForall`.

This property does not hold for embeddings of classical logic into Coq.

Remove anonymous Type fields from camera structures.

See merge request iris/iris!525

These were already removed from the OFE and BI structures, but were left here.

Lemmas about big op on lists for !485

See merge request iris/iris!509

strengthen uPred_mono

See merge request iris/iris!513

More general instance for framing under `<affine>`

See merge request iris/iris!522

Make `iFrame` "less" smart w.r.t. clean up of modalities.

See merge request iris/iris!521

various algebra lemmas

See merge request iris/iris!523

Previously, if would "cleanup" `<affine>` and `□` if the result after framing
is affine and intuitionistic, respectively. This behavior was inconsistent,
since similar "cleanup" was not performed for `<absorbing>` and `<persistent>`.
This MR thus removes this "cleanup" of modalities. It now consistently removes
the modalities `<affine>`, `<absorbing>, `<persistent>` and `□` only if the
result after framing is `True` or `emp`.

Since `iFrame` is already very complicated, and since its performance is
sometimes suboptimal in bigger developments, @jung and I believed doing
fewer "smart" things is better than the alternative, namely performing doing
sophisticated "cleanup" for all modalities, which is presented in
iris/iris!450

document Iris side-effects

Closes #311

See merge request iris/iris!524

Strengthen auth_both_valid and auth_both_frac_valid

See merge request iris/iris!520