add an exhaustivity parameter to Branch constructor in Machine Code to handle C switch default case optimization in case of a singleton enum

ast modifs and additional arrow_taint phase to retrieve which arrow "protects" / "taints" which variable

use ghost variables instead of existential variables to avoid read / write capture problems with variable reuse

more compact assign clauses for state variables, and remove useless memory pack predicates and assertions

more agressive optim propagation in spec in order to remove unecessarry existential variables that were eliminated (helps the solvers)

Array access: solved issues in C backend when basic operations in array access dimensions. Also better handling in EMF, ie further normalization through new equations

- real constants are hidden in Real.ml{i} module

- now properly records the scopes
- only register requested ones