This reverts commit 78ba9509.

This commit implements a generic `iAlways` tactic that is not tied to
`persistently`, `affinely` and `plainly` but can be instantiated with a
variety of always-style modalities.

In order to plug in an always-style modality, one has to decide for both
the persistent and spatial what action should be performed upon introducing
the modality:

- Introduction is only allowed when the context is empty.
- Introduction is only allowed when all hypotheses satisfy some predicate
  `C : PROP → Prop` (where `C` should be a type class).
- Introduction will only keep the hypotheses that satisfy some predicate
  `C : PROP → Prop` (where `C` should be a type class).
- Introduction will clear the context.
- Introduction will keep the context as-if.

Formally, these actions correspond to the following inductive type:

```coq
Inductive always_intro_spec (PROP : bi) :=
  | AIEnvIsEmpty
  | AIEnvForall (C : PROP → Prop)
  | AIEnvFilter (C : PROP → Prop)
  | AIEnvClear
  | AIEnvId.
```

An always-style modality is then a record `always_modality` packing together the
modality with the laws it should satisfy to justify the given actions for both
contexts.

1- Avoid [type_term (eq_refl : @eq Type PROP PROP')] when [PROP] is not given. This has significant performance implications.
2- In th case PROP is given (i.e., only when the tactic is manually used), introduce all the foralls and lets.

This should fix iris-examples.

Reorganize bi.monpred. Add unfolding manual lemmas for monPred_at, and use them in proofmode.monpred. Add big op lemmas for monpred_at.

This used to be done by using `ElimModal` in backwards direction. Having
a separate type class for this gets rid of some hacks:

- Both `Hint Mode`s in forward and backwards direction for `ElimModal`.
- Weird type class precedence hacks to make sure the right instance is picked.
  These were needed because using `ElimModal` in backwards direction caused
  ambiguity.

Use [notypeclasses refine] instead of eapply in iLöb and iNext, to workaround the confusion of apply wrt canonical structures.

This was an oversight in !63.