[ghc-steering-committee] Please review: #454 Custom type warnings

Hi all, As Joachim noted, #454 https://github.com/ghc-proposals/ghc-proposals/pull/454 is extracted from Adam’s previous efforts with the Unsatisfiable constraint https://github.com/ghc-proposals/ghc-proposals/pull/433 proposal (which has since been accepted). In short, it covers an interface for custom warnings, and I can already think of plenty of ways I’d use it. I’d therefore like to recommend acceptance, and discuss any issues or changes. Thanks, Tom