[#566] Add Scenario and PureM to Cleveland exports

Description

Problem: In most cases, we would want to declare a helper function, something to the tune of myScenario :: Monad m => ... -> Scenario m, or for emulated scenarios specifically, myEmulatedScenario :: ... -> Scenario PureM. However, neither Scenario nor PureM are exported from Test.Cleveland, which makes it neccessary to import internals.

Solution: export Scenario and PureM from Test.Cleveland.

This also tweaks haddocks on Scenario, scenario and scenarioEmulated to be a little clearer.

Related issue(s)

Related to #566 (closed)

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