Give `minusInteger` its own implementation.
Previously `minusInteger` used `plusInteger` and `negateInteger`, which
meant it always allocated.  Now it works more like `plusInteger`.

Reviewers: goldfire, hvr, bgamari, austin

Reviewed By: hvr, bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2278

GHC Trac Issues: #12129