Improve how wingman uses evidence

[isovector]

#1517 made Wingman's evidence search much more robust, but I found some more places that GHC can stick dictionaries. This PR adds a few more sources of evidence, namely from GADT constructors and calls to rank-n functions.

That means now we can solve holes like this:

data X f = Monad f => X

fun1 :: X f -> a -> f a
fun1 X = _

Additionally, Wingman can now use evidence that is synthesized when pattern matching on a GADT.

While testing this feature, I realized that Wingman wasn't rewarding generated solutions for using user-bound variables, so that's also included here.

Fixes #1285
Fixes #1550
Fixes #1551