Delegate CRC calculation to fastcrc when it is installed

[ntamas]

This PR delegates the calculation of the CRC checksum in MAVLink messages to the crcmod-plus module when it is installed, falling back to the previous pure Python implementation if needed.

This PR delegates the calculation of the CRC checksum in MAVLink messages to the fastcrc module when it is installed, falling back to the previous pure Python implementation if needed.

In my benchmarks, with real MAVLink traffic from ~1000 drones (recorded by Wireshark), this boost the speed of the MAVLink Python parser from 54K packets per second to 100K packets per second.

Note that I have not declared crcmod-plus as a dependency (not even as an optional extra) yet because I don't know what the preferred solution would be. I would probably declare it in setup.py as an optional dependency and leave it up to the user to decide whether they are okay with an extra C module as a dependency or not. crcmod-plus provides wheels for all the major Python distributions and versions, with an ABI3 interface for future-proofing, so it is a low-risk dependency in my opinion, unlikely to cause any problems for end users of pymavlink.

fastcrc is now declared as a dependency in setup.py in the install_requires section.

[ntamas]

CI jobs are failing because mypy is explicitly prevented from using typing information from site-packages (where fastcrc is installed) so it cannot typecheck anything that involves fastcrc. Either we need to change the mypy config or provide type stubs for fastcrc (which have a slight chance to go out of sync with fastcrc itself in the future).

[tridge]

nice work!

[tridge]

Either we need to change the mypy config or provide type stubs for fastcrc

I'd be happy with either solution. Would crcmod-plus be any better for the type info?

[ntamas]

No, both libraries are typed. The advantages of crcmod-plus would be 1) that it accepts any Sequence[int] out of the box, not only bytes (so we don't need to convert) and 2) that it provides ABI3 wheels so I don't have to rebuild the wheels if a newer minor version of Python is released (but this could be changed easily if one can convince the fastcrc authors to provide ABI3 wheels).

I'll modify the mypy config to look into site-packages for typings.

[ntamas]

All green now.

fixed