Fix (inactive) `MultSlice` rewrite

Fix (inactive) `MultSlice` rewrite

The `MultSlice` rewrite was previously accepting multiplications of
three and more variables even though it was designed for multiplications
of two variables only. Fortunately, the rewrite was not actively used in
the bitvector solver. This commit strengthens the condition in
`applies()` and adds a unit test that checks that x * y * z and x * y do
not get rewritten to the same term.