When a user doesn't need an exact port, but cares more about getting
SOME unused port, they can do:
* Create a socket outside with port=None or port=0.
* Use socket.getsockname()[1] to get the allocated port and pass to the
TCP client somehow.
* Use the created socket to create a TCP server transport.
Use-case: unit-testing embedded software that implements a BLE host. The
controller will be a Bumble controller, connected to the host via a TCP
channel.
* The host will have a TCP-client HCI transport for testing.
* The pytest setup code will allocate the TCP server and pass the port
number to the host.
Also add some unittests with python mock.
This contains Rust wrappers around enough of the Python API to implement Rust versions of the `battery_client` and `run_scanner` examples. The goal is to gather feedback on the approach, and of course to show that it is possible.
The module structure mirrors that of the Python. The Rust API is not optimally Rust-y, but given the constraints of everything having to delegate to Python, it's at least usable.
Notably, this does not yet solve the packaging problem: users must have an appropriate virtualenv, libpython, etc. [PyOxidizer](https://github.com/indygreg/PyOxidizer) may be a viable path there.
