Fixing bv theory after changes to theory bv.why.
The former realization of bv used a bit of sign as the less significant bit which should be the most significant. It did not raise problems because to_int and to_uint were not linked by an axiom in the .why. Now, it is the case, so we rewrote function twos_complement and made some straightforward changes to the existing realization (surprisingly proofs are the same).
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