improve some definitions, simplify some proofs.

The main changes are:

* Function calls in the operational semantics
* Mutually recursive function calls in the axiomatic semantics
* A general definition of the interpretation of the axiomatic semantics  so as
  to improve reusability (useful for function calls, and also for expressions
  in future versions)
* Type classes for stack independent, memory independent, and memory extensible
  assertions, and a lot of instances to automatically derive these properties.
* Many additional lemmas on the memory and more robust tactics to simplify
  goals involving is_free and mem_disjoint
* Proof of preservation of statements in the smallstep semantics

* Some new tactics: feed, feed destruct, feed inversion, etc...
* More robust tactic scripts using bullets and structured scripts
* Truncate most lines at 80 characters

Before, it just ensured that whenever one can split up the memory
into two parts before running the program, the framing part can
be split off when the program is finished. Now it also ensure that
it can be split off at any moment during the execution of the
program.

When we extend to non-deterministic expressions/sequence points we
certainly need this for the Hoare judgment for expressions, as C
allows evaluation of them to interleave. However, for consistency,
and further extensions, it is nice to have this property for
statements too.