Refactor/unify repetitive pattern

This pattern comes up pretty regularly (in Solver.get_all_values, Solver.get_value, ConstraintSet.migrate), should we make a Expression.clone() ?

                if isinstance(foreign_var, Bool):
                    new_var = self.new_bool(name=migrated_name)
                elif isinstance(foreign_var, BitVec):
                    new_var = self.new_bitvec(foreign_var.size, name=migrated_name)
                elif isinstance(foreign_var, Array):
                    # Note that we are discarding the ArrayProxy encapsulation
                    new_var = self.new_array(index_max=foreign_var.index_max, index_bits=foreign_var.index_bits, value_bits=foreign_var.value_bits, name=migrated_name).array