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.
This line is commented out because it's a no-op; `R.foldl === R.reduce` so they already share the same properties. It just looked silly to have a stand-alone JSDoc comment. :grinning: