Add support for Descartes style extreme mutation

[k3rn31]

Descartes is a MT style described in the paper Will my tests tell me if I break this code?. It is hypothetically faster to execute because is more "coarse" and it requires to test less mutations.

As a reference, we can take a look at the PITest third party Descartes plugin.

My idea is to add the single Descartes mutations as "normal" Gremlins mutations, and also add a --descartes flag that will only run Descartes mutations.

Ideas of mutations to implement (TODO: transform to issues):

• void mutation
• nil mutation
• Constant mutation