Remove dispatch from .idx methods

- Remove dynamic dispatch from `reduce.idx`, `reduceRight.idx`, `map.idx` and `filter.idx`.
- Add `foldl.idx` and `foldr.idx` aliases to the docs.
You convinced me. Both options seemed equally confusing at first (if it's uncommented, removing it won't remove the alias). But on a second thought, clarity for a casual reader is more important.