templater: remove __iter__() from _hybrid, resolve it explicitly
The goal is to fix "{hybrid_obj|json}" output.
A _hybrid object must act as a list or a dict as well as a generator of
legacy template strings. Before, _hybrid.__iter__() was assigned for legacy
template, which conflicted with list.__iter__() API.
This patch drops _hybrid.__iter__() and makes stringify/flatten functions
unwrap a generator instead.