Infrastructure changes to improve the test story

[guibou]

With theses change, the code executed to run a rest request is configurable. It means that we can now run test with stub request (not yet done).

For example, we can now write a request function for tests like:

runRequestForTests url headers = pure $ AesonBool True

or

runRequestForTests url headers = throwIO $ VanillaHttpException ...

I'm having second thoughts with this solution, so @gbataille if you can discuss it I'll be really happy.

This solution is great because:

• it does not change a lot of code
• will help us writing real tests asap

This solution sucks because:

• Everything is still working in IO, so effectively our code can do anything which may not be enforced / checked by the test. I don't really care about that one, except for the next point
• Difficult to make the function returns something else than an Aeson Value (typeclass polymorphism does not live well inside structs).
• readFile and putStrLn calls in Krank cannot be mocked.

I can also add something for readFile and putStrLn in the configuration. This way krank will still live in IO, but everything will use customization functions and will be fully testable.

Another solution may be to move to a full "effect" library pattern. However I can see the following effects, currently managed by IO, but that which will need to be detailed:

• Network queries via req (Exactly what I'm changing here)
• Exception throwing in IO
• Parallel computation with mapConcurrently
• file reading with readFile
• output with putStrLn.

It will be a more important change, which can take the following shape:

• a full KrankMonad with all theses operations defined, with one instance for ReaderT KrankConfig IO (i.e. the current implementation) and one for tests (something such as ReaderT (Environment, KrankConfig) (WriterT Outputs (EitherT Exception Identity))).
• A mtl like implementation where each of the listed effect are implemented
• A free monad
• Any other effect library. I'm tempted to play with polysemy, but without real conviction.

[guibou]

Deprecated in favor of #55 which fixes all the discussed problem by introducing a custom monad for effect handling without being too complex to read.