Verify that known-key uniques fit in interface file

Authored by bgamari on Dec 15 2016, 6:00 PM.

Description

Verify that known-key uniques fit in interface file

Here we introduce a debug check asserting that all uniques in
knownKeyNames will fit in the space allowed in the interface file's
symbol encoding.

Test Plan: Validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2845