Implement the inner case-split heuristic, extend case-split to Ors with more than two elements

1) Implement inner-case-split heuristic, that distribute And(..,Or(),..)
in a case-split. It also pushes Not() inside the formula, so
Not(And(a,b)) becomes Or(Not(a), Not(b)) which is then handled by
inner-case-split.

2) Extend regular case-split to work with n-way ors. Or(a, .., m,n) gets
decomposed into a N-alternatives case-split.

Given solutions (Sa, .., Sm, Sn), it recomposes into:
    If(Sa.pre, Sa.term, If(.., If(Sm.pre, Sm.term, Sn.term)))