This reverts commit 78ba9509.

Generic `iAlways` tactic.

See merge request FP/iris-coq!111

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.

In the same style as most of the BI lemmas, e.g. `or_mono`, `and_mono`, ...

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.