Add Input and Output effects

[arybczak]

Proof of concept.

• Effectful.Input.Const as a restricted version of Reader for ensuring that the input parameter doesn't change (static only, since with a dynamic you can change it with interpose).
• Effectful.Output.Array is the correct alternative to Writer when you just want to accumulate values.

And then it turned out that these are generalized by the Coroutine effect, so might as well also include it, it looks like there's no point providing a dynamic Input and Output.

Opinions welcome.

[arybczak]

@tomjaguarpaw perhaps you have an opinion about this, i.e. separate Input/Output vs type synonyms for Coroutine?

[tomjaguarpaw]

If I was doing it again I would have made Stream and Produce newtypes of Coroutine, rather than type synonyms. The error messages would have been better. I don't know that the same reasoning holds for effectful though.