The advantage is that we can directly use a Coq introduction pattern
`cpat` to perform actions to the pure assertion. Before, this had
to be done in several steps:

  iDestruct ... as "[Htmp ...]"; iDestruct "Htmp" as %cpat.

That is, one had to introduce a temporary name.

I expect this to be quite useful in various developments as many of
e.g. our invariants are written as:

  ∃ x1 .. x2, ⌜ pure stuff ⌝ ∗ spacial stuff.

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`.

The tactic was doing something weird and only once used.

- Get rid of wp_finish, which was a hack.
- Write the wp_ tactics for stateful steps in the same style as
  wp_pure, i.e. by taking the context into account.
- Make use of the context K in wp_pure.

Instead of writing a separate tactic lemma for each pure reduction,
there is a single tactic lemma for performing all of them.

The instances of PureExec can be shared between WP tactics and, e.g.
symbolic execution in the ghost  threadpool

For obsolete reasons, that no longer seem to apply, we used ∅ as the
unit.

Also add "Local" to some Default Proof Using to keep them more contained

This patch was created using

  find -name *.v | xargs -L 1 awk -i inplace '{from = 0} /^From/{ from = 1; ever_from = 1} { if (from == 0 && seen == 0 && ever_from == 1) { print "Set Default Proof Using \"Type*\"."; seen = 1 } }1 '

and some minor manual editing