Typechecker plugins: request for review and another workflow question

Hello, I just finished merging HEAD into the branch implementing constraint solver plugins (`wip/tc-plugins`), so things should be fully up to date. For ease of review, I squashed everything into a single commit: https://github.com/ghc/ghc/commit/31729d092c813edc4ef5682db2ee18b33aea6911 could interested folks (I know of SimonPJ, Richard, and Adam) have a look and let me know if things look reasonable? On a related note: I know that we are using phabricator for code review, but I don't know how to use it yet, so please let me know if I can do something to make the review easier. -Iavor