pjdl: introduce decoder for PJON single wire serial link layer

Introduce a protocol decoder which generates 'pjon-link' output from
'logic' input by interpreting the PJDL single wire serial communication
link layer of the PJON protocol stack. This decoder extracts frame
markers, data bytes, as well as their pad/sync decoration. Inspection of
data values, or checks for frame validity remain the responsibility of a
stacked decoder which is shared among several link layer types.

This implementation "violates" the PJDL spec in those places where the
spec is incomplete or vague, and real world traffic would not decode at
all when the strict letter of the spec is applied instead of its spirit.
When in doubt, the decoder implementation errs to the usability side.

Carrier sense detection is incomplete in this version. Data extraction
works for all currently available captures. Recovery from synchronization
loss after glitches is acceptable. Glitch filtering is missing (the spec
is silent on this subject).

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