This PR adds doctests allowing us to write tests for the custom type errors. Having this stuff reified in the test suite means we can iterate on the implementations and give much better QA.
* Ensure wanted constraints are emitted only once.
This is a fix for the "solveSimpleWanteds: too many iterations" problem
that people are running into. In these cases, the plugin is being asked
to solve the same `Member` constraint several times, and generating
a new wanted each time. In doing so, GHC thinks that work was done, so
it will ask the plugin to run again next time. This process diverges,
and produces the error.
Fixes#79
