[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 

[e-acsl] Makefile's target 'install'
[e-acsl] test GMP linkage in non-regression suite ('make tests' now requires GMP >= 4.3.2)
[e-acsl] fixed bugs of GMP compatibility

- [Makefile] better generation of local_config.ml
- [Options] new options for customized code generation (unused yet)

[E-ACSL] use -e-acsl-check for all tests
[E-ACSL] prepare guarding division and module (commented code yet)