Resolves #92

I've opted with mapToFst to denote the function where the result ends up in the first slot,
instead of mapSnd. The names of these are of course an easy fix, if that's what we decide to do.

I've preemptively inlined everything in the module, which should be fine given the size of every function.

I haven added any hlint rules, I think that should be a separate issue if this gets merged.

Checklist:

HLint

• I've changed the exposed interface (add new reexports, remove reexports, rename reexported things, etc.).
  • I've updated hlint.dhall
    accordingly to my changes (add new rules for the new imports, remove old ones, when they are outdated, etc.).
  • I've generated the new .hlint.yaml file (see
    this instructions).

General

• I've updated the CHANGELOG
  with the short description of my latest changes.
• All new and existing tests pass.
• I keep the code style used in the files I've changed (see style-guide
  for more details).
• I've used the stylish-haskell file.
• My change requires the documentation updates.
  • I've updated the documentation accordingly.
• I've added the [ci skip] text to the docs-only related commit's name.