Now a replayed read() will return the response that follows the
matching write().
When calls are made in the same order as they were during recording,
this will have no effect, and the original ordering will be replayed.
However, minor changes to the code should still result in sensible
replay until a new recording can be made.
Calling openConnection will return an open connection or nullptr.
Deleting the connection will close it.
SerialPort now uses this under the hood, while still presenting
the QSerialPort-compatible interface.
This initial commit is designed to change as little existing code as possible.
Once regression tests are in place that can play back previously recorded
data, we can move on to more significant changes.
