update core-spec for GRefl and re-factored Refl

Authored by ningning on Jul 26 2018, 4:20 PM.

Description

update core-spec for GRefl and re-factored Refl

Ticket Trac #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