Add close() method to Channel and ByteChannel

[benoitc]

Summary

• Add close() method to Channel and ByteChannel Python classes
• Add context manager support (with statement) for automatic cleanup
• Uses existing channel_close() C function exposed via new erlang._channel_close() binding

Usage

# Explicit close
channel.close()

# Context manager (auto-close on exit)
with Channel(ref) as ch:
    for msg in ch:
        process(msg)
# channel automatically closed

# Erlang detects closure
case py_channel:recv(Channel) of
    {ok, Data} -> process(Data);
    {error, closed} -> channel_was_closed
end.