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

Before submitting your PR, check that you've:

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

After submitting your PR:

• Update the Changelog.md file with a link to your PR.
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts).