Generate appropriate Handler and Widget synonyms for polymorphic sites

[maurges]

A fix for #1629

I've looked into #1630 which is an attempt to fix the same issue, and mine does a similar thing, but doesn't break declarations without typevars. Also mine has tests.

The tests show that the code like in the docs wouldn't compile before my change, but compile as expected with it.
I wasn't sure how to create tests for template haskell, so the tests fail at compile time.

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[snoyberg]

Awesome changeset, thanks!