[e-acsl] update todo-list

[e-acsl] tests now independent of GMP version (but GMP still required)

[e-acsl] all tests are now executed with gcc
[e-acsl] fix bug with integer division and modulo
[e-acsl] untested support of most logic labels
[e-acsl]

[e-acsl] support of \at for stmt labels

[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] 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

- 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