fix form of rewrites

[dwightguth]

This should fix the issue with the representation of rewrites in kore.

[ttuegel]

@grosu realized, that this translation is not correct; instead it should be:
for rewrite rules: \rewrites(\and(\and(requires, ensures), L), R)
for function rules: \implies(\and(requires, ensures), \equals(L, R))

[dwightguth]

Since we decided to disallow ensures in axioms, I will merge this. I will do a followup PR that existentially quantifies RHS variables.