uart: add FRAME Python annotation, communicate frame validity

uart: add FRAME Python annotation, communicate frame validity

Internally keep track of the UART frame's validity. Emit a FRAME Python
annotation for aborted as well as for completed frames. This obsoletes a
TODO comment in the STOP bit code path.

This annotation also spans the complete frame's length, including start
and parity and stop bits, which the DATA annotation doesn't cover.
Stacked decoders can individually decide whether to strictly reference
the mere data bits section or the complete UART frame which happened to
communicate the data value.