C backend : preparatory work

[denismerigoux]

Implements groundwork for #204.

High-level changes

This PR performs some cleaning of the generated Python code, and creates a prototype for closure conversion that is not yet working. To make closure conversion work properly, we need more structure at the level of Lcalc. Before brining this structure to Lcalc, we need to clean up the Dcalc binded representation and factor things up.

Changelog

• Implements some optimizations and prettyfication of the Scalc backend
• Implement closure conversion prototype
• Make the scope let-binding structure default in Dcalc
• Bring the scope let-binding structure down to LCalc
• Bring the scope let-binding structure down to SCalc
• Debug closure conversion and minimize the amount of conversions.

[denismerigoux]

I'm merging this now since it"s already quite big and I will do more refactoring before implementing the closure conversion proper.