Rewrite coercionRole. (#9233)

Summary:
coercionRole is now much more efficient, computing both the coercion's
kind and role together. The previous version calculated them separately,
leading to quite possibly exponential behavior.

This is still too slow, but it's a big improvement.

Test Plan: Evaluate by running the "minimized" test from the Trac ticket.

Reviewers: simonpj, austin

Subscribers: simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D73