Add a backend for Silicon Labs CP2110/4 HID-to-UART bridge.

[Flameeyes]

These two chips implement UART access via a HID protocol, which can be
implemented purely in user space. The protocol is documented by Silicon
Labs AN434:
https://www.silabs.com/documents/public/application-notes/AN434-CP2110-4-Interface-Specification.pdf

The backend is implemented based on cython-hidapi
(https://github.com/trezor/cython-hidapi), making it OS-independent, if a
bit awkward.

[zsquareplusc]

could you move the complete code into serial/urlhandler/protocol_cp2110.py? I don't think that anyone would need to import serial.cp2110.

And there should probably be a note in documentation/url_handlers.rst mentioning the handler and that the hidapi module needs to be installed separately (i would not like to install that by default, but as option would be possible, via extras_require={'cp2210': [hidapi']} in setup.py).

I've also used HID here and did my own low-level access:
https://github.com/zsquareplusc/python-msp430-tools/blob/master/msp430/bsl5/hid.py#L84
hoever that does not (yet) support MacOSX

[Flameeyes]

Thanks for the review!

Code moved and documentation and extras_require added.

I have implemented raw HID support myself before, and honestly it's sometimes nicer particularly with the ability to have a more rational path compared to hidapi. But the problem with that is that implementing select/timeout on it is more fragile than HIDAPI, which is why I'm not terribly keen on doing that here.

[zsquareplusc]

Thank you