add an extra ASTNode pass

[gdementen]

the goal is to "compile" expressions to another representation which:

• has an uniform interface, so that it can be walked/traversed recursively without having to define a traverse method on most expression classes.
• has not its eq method overloaded so that I can use "expr in d" (dict.contains(k) calls k.eq) so that I can store expressions in a dict to check whether they occur more than once

Questions:

• use ASTNode class in numexpr as is?
• would an uniform interface make things cleaner for current functionalities (mainly as_string, collect_variables and simplify)?

[gdementen]

done (devel branch is merged)