[e-acsl] support of field accesses in logic
[e-acsl] support of array accesses (yet missing guards) in logic
[e-acsl] support of pointer dereferences (yet missing guards) in logic
[e-acsl] support of \null in logic

[e-acsl] support of assumes without ensures in stmt contracts
[e-acsl] partial support of function contracts (the same subset than stmt contracts)
[e-acsl] additional test for nested code annotation
[e-acsl] code refactoring: visit.ml splitten into several compilation units

first step for support of statement/function contracts (statement ensures could work but yet untested)

[e-acsl] fixed bug for negative modulos
[e-acsl] full reimplementation of environments (only one now, and it is persistent)
[e-acsl] support of lazy operators &&, || and ==>
[e-acsl] tests now execute the generated C code and check that no runtime assertion is raised

[e-acsl] add guard before divisions and modulos

- update TODO-list
- location for new constructs whenever possible
- additional warnings when guards are missing