Current versions of Alex don't seem to produce as many warnings any
more.

In order to silence a warning and to avoid overlong lines, I've taken
the liberty of refactoring 'tok_num'.

Test Plan: ./validate

Reviewers: bgamari, simonmar

Reviewed By: simonmar

Subscribers: erikd, rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5319