WIP Create new Reader interface

[wizzomafizzo]

This PR separates the existing libnfc code into a separate module and exposes it as a new Reader interface. This will have some really nice benefits:

• New types of "readers" can be introduced without any changes to the core code such as the generic serial reader
• TapTo can be compiled without libnfc (in case Windows doesn't end up supporting it)
• More than one reader can be connected at once

I think it just generally makes the daemon process much easier to read and deal with as well.

The PR is still incomplete. Before it's merged I still need to:

• [x] Move the remaining libnfc code from the reader detection section and tokens module to the libnfc module
• [x] Add support for multiple readers at once and introduce new logic for handling simultaneous scans
• [x] Thoroughly test the daemon loop and make sure I haven't broken any behaviour with that (I probably have)

[wizzomafizzo]

all tests passing finally! this code now works exactly how it did before 🥳

@asturur sorry for rewriting so much of the code. i tried for a couple of days to edit the existing way and make it work but it was so complex to do it with the way i am using channels now. at least i have no lost any functionality in the rewrite

next i will add some basic support for multiple readers at once, and then i'll stick a fork in this

[wizzomafizzo]

ok, all good. no regressions as far as i can tell

• now multiple readers can be attached at once
• pn532_uart and acr122_usb readers will continue to be auto-detected
• a new "file" reader exists which can use the contents of a monitored file as a virtual reader, can be enabled by setting "file:" as a connection string

scanning behaviour should be exactly the same as previously, following the test results in the docs