Summary:
Add doctest examples for the three standalone functions defined in
Data.Functor:

  * Data.Functor.$>
  * Data.Functor.<$>
  * Data.Functor.void

This is part of a larger plan to add examples for the functions in
base, and to eventually enable automatic testing of them.

Reviewers: austin, hvr, ekmett

Reviewed By: austin

Subscribers: hvr, ekmett, thomie, carter, ezyang, simonmar

Differential Revision: https://phabricator.haskell.org/D352