Add elm-review, remove dependencies and add missing exposed types

[jfmengels]

Adds elm-review with a stripped down review configuration. Feel free to uncomment more rules when you see fit (there are a bunch of problems waiting to be uncovered to be honest).

I noticed some unused dependencies (including one that is only unused when we first remove unused functions). Also, in Elm.Let, a lot of functions were referring to a type that was not exposed. This will cause a minor version bump. The build now fails because that type needs some documentation, which I'll let you add.

[jfmengels]

I resolved the conflicts on the PR

[mdgriffith]

Thank you!