rename fmap to map

> < Ralith> someone should rename fmap to map already

To still allow disambiguation of `List.map` vs. `Functor.map` on `List` arguments, I replaced the former entirely by moving the `Functor` instance of `List` into `Prelude/List.idr` (and importing `Prelude.Functor` there). Other than this, nearly the whole change was made by global search-and-replace.