Core of XDNA diff mechanism
* Removing debug code from XDNA class
* Remove unnecessary json import
* single quotes -> double quotes for function descriptions
* Function description for XDNA::signatures_diff()
First version of new file format

Some last-minute amendements were made to the file format layout and naming, but this is all probably temporary anyway.
The new data it "saves" (the framerate) goes unused right now.