Implement Datetime serialization

Simplified serialization of trading bars

Use first instrument returned (best match) instead of last

Implement commission reports

Drop support for versioned message handlers (YAGNI), and enforce handler signature.

Implement executions

Drop mypy36 from the test list - no added value

Check that all functionality is actually included in the client.

Implement orders (market&limit)

Implement wrap_immediate_future

Implement alternative representations of float(None) and int(None)

Reuse the test instrument, to more closely simulate actual behaviour.

Simplified instrument (de-)serialization

Rewrite outgoing serialization to be more similar to incoming serialization

Implement positions.