embedded-hal based PHY

Would be nice to inclue a generic PHY built ontop of the embedded-hal traits. I think this should be possible with the e-h 1.0 traits at least.

There will be problems as soon as more dynamic baudrate handling will become necessary (e.g. automatic baudrate detection). embedded-hal currently does not expose a generic interface for baudrate management, see rust-embedded/embedded-hal#79.