Added a "Contracts" pass, after inlining, taking care of the
contracts of the nodes called in the body of a node. This pass
"inlines" the code and assume/guarantee parts of these subcontracts.
The "Sigali" pass both generates the sigali ("z3z") code and add the call to
the controller (which is a node generated further by the sigali tool).
Therefore this pass has been included into the mls compiler, and removed
from the targets (a "z3z" dummy target has been kept for backward compatibility
reasons).
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