Support tuple -> list conversion

Useful whenever tuples are not supported by pythran: just convert them
to a list (if it is meaningful). For instance to iterate over a tuple...
Huge rework of itertools

* indent everything properly
* use std::forward to avoid unnecessary copies
* do not store the value computed by the operator*

As a result, izip and imap are really faster (x2 on a bench here), I did
not implement the same optimizations for ifilter and generate. Yet.