Summary:
`pprParendType` was missing an explicit case for
`EqualityT`, which caused it to fall through to a catch-all case
that invokes `ppr`. But `ppr` itself does not have a case for a
partial application of `EqualityT`, so //it// falls back to
`pprParendType`, resulting in an infinite loop!

The fix is simple: add a case for `EqualityT` in `pprParendType`.
While I was in the neighborhood, I removed the catch-call case in
`pprParendType` to make this sort of mistake less likely to happen
in the future.

Test Plan: make test TEST=T15985

Reviewers: bgamari, monoidal, simonpj

Reviewed By: monoidal, simonpj

Subscribers: rwbarton, carter

GHC Trac Issues: #15985

Differential Revision: https://phabricator.haskell.org/D5403