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Major changes:
* A data structure to collect locked addresses in memory.
* Operations to lock and unlock addresses.
* Remove [ctree_Forall] and express it using [Forall] and [ctree_flatten]. This
  saves a lot of lines of code.
* Add a [void] value. This value cannot be typed, but will be used as a dummy
  return value for functions with return type [void].

Minor changes:
* Various deciders in preparation of the executable semantics.
* Improve naming and notations.
* Remove obsolete stuff.

The refactoring includes:
* Use infix notations for the various list relations
* More consistent naming
* Put lemmas on one line whenever possible
* Change proofs into one-liners when possible
* Make better use of the "Implicit Types" command
* Improve the order of the list module by placing all definitions at the start,
  then the proofs, and finally the tactics.

Besides, there is some new machinery for proofs by reflection on lists. It is
used for a decision procedure for permutations and list containment.