react and split WDs appropriately

[nfrisby]

As of 13c6608c0082fb98c1c162cb1568034fc74ba2b3, the plugin:

1. reacts Gs
2. reacts Ds with fixed G cache
3. simplifies Ws and WDs with fixed G cache

This Issue is to refine that to:

1. reacts Gs
2. reacts WDs and Ds with fixed G cache; split any changed WDs into an unchanged W and changed D
3. simplifies Ws with fixed G cache

See https://gitlab.haskell.org/ghc/ghc/wikis/plugins/type-checker/notes for more context.

[nfrisby]

While I'm at it, this Issue would be a good time to

• de-duplicate the code in Plugin.hs for Deriveds and Wanteds as much as possible
• determine when to generate Derived vs Wanted for "derived equalities" arising when simplifying Wanteds (I thought it should be Wanted, but that caused tests to fail; maybe splitting will fix that)