Add usage examples at the top of the user-facing modules

[ocramz]

• [x] #335
• [x] #116
• and optionally #275 (though it requires hspec-wai )

Best would be to use doctest in the Haddock documentation, see e.g https://quchen.github.io/generative-art/generative-art-0.1.0.0/src/Geometry.Core.html for a cool example