flatten handles CallStacks

[parsonsmatt]

Fixes #23

This PR adjusts flatten so that it appropriately handles CallStacks. Before this, you could get into trouble like this:

flatten (AnnotatedException [callStackAnnotation] (AnnotatedException [callStackAnnotation] TestException))
    = AnnotatedException [callStackAnnotation, callStackAnnotation] TestException

After this PR, the above will evaluate to:

flatten (AnnotatedException [callStackAnnotation] (AnnotatedException [callStackAnnotation] TestException))
    = AnnotatedException [mergeCallStacks callStackAnnotation callStackAnnotation] TestException

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• Bumped the version number.
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• Added @since declarations to the Haddock.
• Ran stylish-haskell and otherwise adhered to the style guide.

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