Expressions like the following will now typecheck:

```
    data A x = A deriving Show

    class ToA a x where
       toA :: a -> A x

    instance ToA Integer x where
       toA _ = A

    main = print (toA 5 :: A Bool)
```

The new defaulting rules are

Find all the unsolved constraints. Then:

* Find those that have exactly one free type variable, and partition
  that subset into groups that share a common type variable `a`.
* Now default `a` (to one of the types in the default list) if at least
  one of the classes `Ci` is an interactive class

Reviewers: goldfire, bgamari, austin, mpickering, simonpj

Reviewed By: bgamari, simonpj

Subscribers: mpickering, simonpj, goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D2822