Various fixes to type-tidying

This MR was triggered by #24868, but I found a number of bugs
and infelicities in type-tidying as I went along.  Highlights:

* Fix to #24868 is in GHC.Tc.Errors.report_unsolved: avoid
  using the OccNames of /bound/ variables when tidying /free/
  variables; see the call to `tidyAvoiding`.  That avoid the
  gratuitous renaming which was the cause of #24868. See
     Note [tidyAvoiding] in GHC.Core.TyCo.Tidy

* Refactor and document the tidying of open types.
  See GHC.Core.TyCo.Tidy
     Note [Tidying open types]
     Note [Tidying is idempotent]

* Tidy the coercion variable in HoleCo. That's important so
  that tidied types have tidied kinds.

* Some small renaming to make things consistent.  In particular
  the "X" forms return a new TidyEnv.  E.g.
     tidyOpenType  :: TidyEnv -> Type -> Type
     tidyOpenTypeX :: TidyEnv -> Type -> (TidyEnv, Type)