Better dead code elimination

[RenaudFondeur]

Add a better class for dead code elimination allowing the suppressions of almost all of the hidden hundreds warnings related to unused variables.

Also makes some change in Slang:

• Move some Slang related transformation before dead code elimination as they are generating dead code.
• Fix some AST transformations that made exploration/operation incorrect (by breaking parents' links or using the same node in multiple places).

Follow up task:

• The transformation moved are in expandsAST. They are now duplicated (to avoid breaking existing tests), the second call does nothing.
• Currently Slang doesn't know how to determine if a selector has a side effect, so the new class uses a list of simple hard coded selectors (- + = > ...), it would be nice for Slang to be able to determine this automatically and support more selectors.
• The tests of the old algorithm have not been transferred to the new yet.