Add Data.List.uncons

Summary:
As discussed in
http://www.haskell.org/pipermail/libraries/2014-July/023314.html and
submitted at #9550.

Test Plan: Submit to phab, see what happens.

Reviewers: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D195

GHC Trac Issues: #9550
Revert "Remove max_bytes_used test from haddock test cases"

This reverts commit 8c427ebe9c34b9bcecd41fb0584d24989c00ffe6.

Sorry for the noise, but I need to practice my HIW talk, which will
involve some live development...