This is the renaming that @simonpj requested:
```
· zipOpenTCvSubst  -> zipTvSubst   (It only deals with tyvars)

· zipOpenTCvSubstCoVars -> zipCvSubst   (it only deals with
covars)

· zipOpenTCvSubstBinders ->  zipTyBinderSubst  (it only deals
with TyBinders, not covars)
```
plus the `mk` variant.

Test Plan: ./validate

Reviewers: simonpj, goldfire, austin, bgamari

Subscribers: thomie, simonpj

Differential Revision: https://phabricator.haskell.org/D1853

GHC Trac Issues: #11371