Preparing serialization

Frogot that rbx MUST be preserved :/

Write a macro allowing to "declare" method header's argument list

- Adds str & repr to RPNIterExpr
- Adds richcompare to RPNExpr
- Adds tests

After mutation some token has non zero initialized useless data that should not
be copied in state.

Starting with __len__
Next we have to implement token acces as a sequence in RPNExpr, but to do
this we have to implement a module allowing token access from python.