Add Functor, Applicative, Monad instances for First, Last

This was proposed in 2011 [1] with no serious objections although wasn't
implemented until it was again mentioned in 2014 [2].

[1] http://www.haskell.org/pipermail/libraries/2011-January/015552.html
[2] http://www.haskell.org/pipermail/libraries/2014-June/023228.html