The proof now uses the stronger notion of memory permutation instead of a more
general memory refinement. We have also proven that memory permutations are
symmetric.

Also better error messages if a constant expression is expected.

Both the operational and axiomatic semantics are extended with sequence points
and a permission system based on fractional permissions. In order to achieve
this, the memory model has been completely revised, and is now built on top
of an abstract interface for permissions.

Apart from these changed, the library on lists and sets has been heavily
extended, and minor changed have been made to other parts of the prelude.

The development now corresponds exactly to the FoSSaCS 2013 paper.
Also, the prelude is updated to the one of the master branch.

* Define the standard strict order on pre orders.
* Prove that this strict order is well founded for finite sets and finite maps.
  We also provide some utilities to compute with well founded recursion.
* Improve the "simplify_option_equality" tactic to handle more cases.
* Axiomatize finiteness of finite maps by translation to lists, instead of by
  them having a finite domain.
* Prove many additional properties of finite maps.
* Add many functions and theorems on lists, including: permutations, resize,
  filter, ...

Most interestingly:
* Use [lia] instead of [omega] everywhere
* More many generic lemmas on the memory to the theory on finite maps.
* Many additional list lemmas.
* A new interface for a monad for collections, which is now also used by the
  collection tactics.
* Provide an additional finite collection implementation using unordered lists
  without duplicates removed. This implementation forms a monad (just the list
  monad in disguise).

The following things have been changed in this revision:

* We now give a small step semantics for expressions. The denotational semantics
  only works for side-effect free expressions.
* Dynamically allocated memory through alloc and free is now supported.
* The following expressions are added: assignment, function call, unary
  operators, conditional, alloc, and free.
* Some customary induction schemes for expressions are proven.
* The axiomatic semantics (and its interpretation) have been changed in order
  to deal with non-deterministic expressions.
* We have added inversion schemes based on small inversions for the operational
  semantics. Inversions using these schemes are much faster.
* We improved the statement preservation proof of the operational semantics.
* We now use a variant of SsReflect's [by] and [done], instead of Coq's [now]
  and [easy]. The [done] tactic is much faster as it does not perform
  inversions.
* Add theory, definitions and notations on vectors.
* Separate theory on contexts.
* Change [Arguments] declarations to ensure better unfolding.

improve some definitions, simplify some proofs.