Add heap_lang lib for "invariant locations": locations with a (pure) invariant attached to them

See merge request iris/iris!289

Make use of `▷^` notation in its definition.

See merge request iris/iris!428

Fix notation issues in #302 and add some missing "operator sections" for (bi)entailment

Closes #302

See merge request iris/iris!409

This works because there is already a `Reserved Notation`.

Fixed a spacing issue in metatheory.v

See merge request iris/iris!427

Fix #302, including their ASCII variants.
- Don't use quotes `'` that are not surrounded by spaces.
- However, notation `'(⊢@{' PROP } )` prevents parsing `(⊢@{PROP} Q)` using the
`⊢@{PROP} Q` notation.

To fix that, we force left-factorization: we add a notation for `'(⊢@{' PROP }
Q )`, defined to coincide with '⊢@{' PROP } Q but which can be left-factored
with `( '⊢@{' PROP } )`.

- Add left and right operator sections for (bi)entailment
- Add tests.

Also do all of the above also for ASCII notations, except for operator sections,
which seem to require more discussion.

add wp proof rule for recursive functions

See merge request iris/iris!425

Lemmas stating that big ops are commuting with op/sep.

See merge request iris/iris!422

Document `%H` pattern.

Closes #305

See merge request iris/iris!423

Fix iIntros on forall

Closes #307

See merge request iris/iris!424