uart: Improve robustness of query API result processing

uart: Improve robustness of query API result processing

Since either of the UART signals (RX, TX) is optional, and in the
absence of Decoder.wait() conditions that "will never match", we cannot
construct a constant layout. Instead we need to explicitly keep track of
which item in the list of wait conditions corresponds to which signal.

Once the index in the list of wait conditions is known, inspection of
samples can depend on the Decoder.matched[] attribute. Before this
change, redundant reached_bit() checks kept us from processing samples
that should not have been inspected. Tests pass before and after this
very commit.