Add test suite and rehabilitate CI

[sjshuck]

Related to #155 but distinct.

[sjshuck]

CI added in #163.

[sjshuck]

• Implement Haskell CI #166
• Test that onError right biases exceptions
• Test strictness properties of writer ~(a, w) = do tell w; return a per @mniip

[mniip]

To clarify, there's a bunch of strictness and/or thunk creation properties around strict/lazy state and writer, and every lazy pattern match in the source is indication that something fishy is going on, possibly needing testing. Not just this specific definition.