WIP: Formal Power Series

Formal power series implementation using lazy recursion.
I don't think we should subclass Stream from Expr.  The Stream is a container (just like Tuple) to represent a collections.abc.Iterator, maybe we should add also \__getitem__ interface too.  You should also think about the efficiency.  Do we want to recompute Stream again for first 10 terms?

Probably it's not a right way to implement Stream's arithmetics (e.g. subclass from Expr and override \__add__ and so on).  But keep going on as-is, lets build something working first, e.g. `Stream(sin(x))*Stream(cos(x))`...