Fix bug in string formatting

Plain " in a string should be slash-protected.
Add support for the ord intrinsic
Fix return_alias implementation
Add replace method from str module

This unlocks euler22 and eulereuler42 :-)
Rework class hierarchy in intrisic.py

So that function intrinsic can also have a combiner.
PEP8 compliancy
Optimizations using iterators :
* Analyse of potential iterators, even in user-created functions.
* Transformation of List Comprehensions intos map(...) when possible
* Transformations of intrinsics into iterators (ex : map -> imap)
* Transformation of remaining List Comprehensions into Generator Expressions
* Docstring with each transformation
* Test cases