WIP: Formal Power Series

Formal power series implementation using lazy recursion.
Some cases looks like you don't understand well which cases and what's wrong with rsolve.

No doubts - rsolve has algorithms to find solutions for such equations.  Do we have a bug with some rsolve_* method or we indeed have a need to implement some new algorighm for rsolve?  In the later case - we should improve the recurr module.