The attached patch introduces async_chat.set_terminator_str and
async_chat.push_str methods, accepting a string and an encoding. It also
modifies the test suite and stdlib to use the _str versions, where
appropriate, adds tests for the new methods, and adds documentation.

The patch also includes two things not materially related to its main
purpose: a lot of docstrings, and a new async_chat.push_iterable method
allowing the use of an iterable as an async_chat data producer (along
with test and documentation). I figure docstrings are always justified,
and push_iterable is both short and highly useful.