Early-terminating left folds via Either.
Rename `destructure` to `uncons`.

By analogy with `cons`. This is a great name, following Scala and
Parsec.
One code path for left folds.
Compose rather than writing .Right/etc.
Test early termination of left folds.
Bump `Memo` for public `map`.
Early-terminating right folds via Memo.
Test early termination of right folds.
Merge pull request #44 from robrix/early-terminating-folds

Early terminating folds