Serialize docstrings to ifaces, display them with new GHCi :doc command
If -haddock is set, we now extract docstrings from the renamed ast
and serialize them in the .hi-files.
This includes some of the changes from D4749 with the notable
exceptions of the docstring lexing and renaming.
A currently limited and experimental GHCi :doc command can be used
to display docstrings for declarations.
The formatting of pretty-printed docstrings is changed slightly,
causing some changes in testsuite/tests/haddock.
Test Plan: ./validate
Reviewers: alexbiehl, hvr, gershomb, harpocrates, bgamari
Reviewed By: alexbiehl
Subscribers: rwbarton, thomie, carter
Differential Revision: https://phabricator.haskell.org/D4758