chore: move pure code into shared modules ported from upstream

[Pavel Feldman (pavelfeldman)]

• Move pure code (no asyncio, greenlet or I/O) into modules named after their upstream files, so that the old and new implementations can share it: _serializers.py, _time.py, _timeout_settings.py, _mime_type.py, _headers.py, _locator_utils.py, and error parsing in _errors.py.
• The serializer takes a callback for handles, as upstream does.
• Each ported module names its upstream file in its docstring.