Add non-deterministic expressions with side-effects.

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.