We directly build up the correct AvailInfos rather than generating
lots of singleton instances and combining them with expensive calls to
unionLists.

There are two other small changes.

* Pushed the nubAvails call into the explicit export list
  branch as we construct them correctly and uniquely ourselves.
* fix_faminst only needs to check the first element of the export
  list as we maintain the (yucky) invariant that the parent is the
  first thing in it.

Reviewers: simonpj, austin, bgamari

Reviewed By: simonpj, bgamari

Subscribers: simonpj, thomie, niteria

Differential Revision: https://phabricator.haskell.org/D2657

GHC Trac Issues: #12754