Ticket #15192 introduced the generalized reflexive coercion `GRefl` and
nominal reflexive `Refl`, and removed `CoherenceCo`. Update core-spec
accordingly.  Not sure about notations though; suggestions on more
concise notations would be great.

Test Plan: Read core-spec.pdf

Reviewers: goldfire, bgamari

Reviewed By: goldfire

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4984