Allow mutation of original source code, instead of tests

[ggrieco-tob]

If I understand correctly, necessist will always mutate tests. However, we could also mutate the original source code and re-run the original tests to check if they pass. In that case, there is an issue with the tests.

[smoelius]

@ggrieco-tob I'm attaching a future-enhancement label to this issue.

Necessist is designed specifically to mutate tests. Moreover, ideas I have for enhancing it take it further down that path (see the Theoretical Motivation section of the readme, for example). Also, there is already a good general-purpose, multi-language mutation testing tool, universalmutator. For these reasons, I don't foresee Necessist mutating non-test source code in the near future.