More comments on role subtyping, unsoundness fix.
- We incorrectly allowed subroling on injective data in some cases. There is now a test to check for this case, and a Note.
- More commentary on how the subtyping with roles works.
Signed-off-by: Edward Z. Yang <firstname.lastname@example.org>
Test Plan: validate
Reviewers: goldfire, austin, simonpj, bgamari
Differential Revision: https://phabricator.haskell.org/D3222