Remove unreachable internal code

- partially
- checkMethod with 4 arguments
Merge pull request #310 from fson/remove-unreachable

Remove unreachable internal code
first batch of sigs
Merge pull request #314 from buzzdecafe/master

first batch of sigs
more sigs
Merge pull request #315 from buzzdecafe/master

more sigs [ci-skip]
sigs sigs everywhere
Merge pull request #317 from buzzdecafe/master

sigs sigs everywhere [ci-skip]
more sigs, almost done
Merge pull request #318 from buzzdecafe/master

more sigs, almost done [ci-skip]
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.
first draft sigs done
Merge pull request #320 from buzzdecafe/master

first draft sigs done
Merge pull request #316 from fson/remove-idx-dispatch

Remove dispatch from .idx methods