Add export lists to some modules.

Summary:
This makes it easier to see what is exported, and allows us to
add non-exported top-level names.

Reviewers: hvr, austin, ezyang

Reviewed By: ezyang

Subscribers: ezyang, carter, thomie

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D551

GHC Trac Issues: #9852