Update core spec to reflect changes to Core
Commit message:
Update core spec to reflect changes to Core.
Key changes:
* Adds a new rule for forall-coercions over coercion variables, which
was implemented but conspicuously missing from the spec.
* Adds treatment for FunCo.
* Improves commentary (including restoring a Note lost in
03d4852658e1b7407abb4da84b1b03bfa6f6db3b) in the source.
No changes to running code.
Would like review from @simonpj.