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Other heap_lang related instances in that file too. Now, the tactics
file only contains stuff related to actual tactics.

We add a specific constructor to the type of expressions for injecting
values in expressions.

The advantage are :
- Values can be assumed to be always closed when performing
  substitutions (even though they could contain free variables, but it
  turns out it does not cause any problem in the proofs in
  practice). This means that we no longer need the `Closed` typeclass
  and everything that comes with it (all the reflection-based machinery
  contained in tactics.v is no longer necessary). I have not measured
  anything, but I guess this would have a significant performance
  impact.

- There is only one constructor for values. As a result, the AsVal and
  IntoVal typeclasses are no longer necessary: an expression which is
  a value will always unify with `Val _`, and therefore lemmas can be
  stated using this constructor.

Of course, this means that there are two ways of writing such a thing
as "The pair of integers 1 and 2": Either by using the value
constructor applied to the pair represented as a value, or by using
the expression pair constructor. So we add reduction rules that
transform reduced pair, injection and closure expressions into values.
At first, this seems weird, because of the redundancy. But in fact,
this has some meaning, since the machine migth actually be doing
something to e.g., allocate the pair or the closure.

These additional steps of computation show up in the proofs, and some
additional wp_* tactics need to be called.

- Removing head of list of observations after each reduction step in definition of wp
- Adding support for observations to state_interp and world
- Applying Ralf's suggestions to previous commit (e.g. replacing /\ and -> with unicode characters)

This restricts CAS to only be able to compare literals with literals, NONEV with NONEV and NONEV with SOMEV for a literal.

This reverts commit 913059d2.

This reverts commit 849abb8d, reversing changes
made to 8de0b894.  The merge was done
accidentally, before review happened.

I saw no need for `stuckness_flip`: strong atomicity always works,
while weak atomicity works only for expressions that are not stuck.
Since this seemed unclear, I split lemma `wp_atomic'` up into
`wp_strong_atomic` (parametric in the WP's `s`) and `wp_weak_atomic`
(not). The proof mode instance is stated in terms of the derived rule
`wp_atomic` (parametric in `s`).

This class, in combination with `TCForall`, turns out the useful in
LambdaRust to express that lists of expressions are values.

We used to normalize the goal, and then checked whether it was of
a certain shape. Since `uPred_valid P` normalized to `True ⊢ P`,
there was no way of making a distinction between the two, hence
`True ⊢ P` was treated as `uPred_valid P`.

In this commit, I use type classes to check whether the goal is of
a certain shape. Since we declared `uPred_valid` as `Typeclasses
Opaque`, we can now make a distinction between `True ⊢ P` and
`uPred_valid P`.