Use a patch instead of CtMethod to represent an amplified test method

[danglotb]

The idea is to keep a set of modification from the original test method. The patch could have also ancestors patches, that must be apply to the original test method before applying the patch itself. In this way, we can easily keep a track of the history and the changes that has been chained to obtain the amplified test.

[danglotb]

This feature would allow us to implement easily crossover, that seems to be a key-feature of GI.