Add defaults `foldl` to/from `foldr`.
Move `index` to `FoldableLL`.
Move `findIndex` and similar to `FoldableLL`.

`findIndices` and `elemIndices` now return /lazy/ lists instead of
arbitrary collections. Arbitrary collections would require moving them
to `UnfoldableLL`, which would be IMHO a bit confusing.
Move `isPrefixOf` to `FoldableLL`.