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Summary:
Currently, `DeriveAnyClass` has two glaring flaws:
* It only works on classes whose argument is of kind `*` or `* -> *` (#9821).
* The way it infers constraints makes no sense. It basically co-opts the
algorithms used to infer contexts for `Eq` (for `*`-kinded arguments) or
`Functor` (for `(* -> *)`-kinded arguments). This tends to produce overly
constrained instances, which in extreme cases can lead to legitimate things
failing to typecheck (#12594). Or even worse, it can trigger GHC panics
(#12144 and #12423).
This completely reworks the way `DeriveAnyClass` infers constraints to fix
these two issues. It now uses the type signatures of the derived class's
methods to infer constraints (and to simplify them). A high-level description
of how this works is included in the GHC users' guide, and more technical notes
on what is going on can be found as comments (and a Note) in `TcDerivInfer`.
Fixes #9821, #12144, #12423, #12594.
Test Plan: ./validate
Reviewers: dfeuer, goldfire, simonpj, austin, bgamari
Subscribers: dfeuer, thomie
Differential Revision: https://phabricator.haskell.org/D2961
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