Add unfold.
Right unfolds return the state to the right.
Docs.
Rename unfold to unfoldRight.
Add unfoldLeft.

With notes about performance, infinite streams, and recursion.
Merge pull request #42 from robrix/unfold

Unfolds
NilLiteralConvertibleConformance.
Document the constructors.
Fix up a documentation comment.
Use `nil` instead of `.Nil`.
Merge pull request #43 from robrix/nil-literal-convertible

Nil literal convertible
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