a bench prototype for von koch, with the relevant script.
Simplify lilis.ml.
New organisation of compression, change in complete transformation.

Now, a transformation is used as complete by transforming any undefined symbols in Nil
Misc call optimisations in the engine.
Intermediary benchs
Add empty to the interface, get rid of Stream.
Simplify transformations to/from list, whithout performances loss.
little fixup for bench results.
Implement the graphical part of the definitions and optionnal arguments.

- Make lsystem and rules polymorph on their return type.
- Add post rule manipulation functions.
- Add default definitions, injected into parsed lsystems.
- Change the default engine function so that it use post rules instead of decompressing.
- Adapt graphical backends to work with post rules.