parallel: rephrase handling of data lines, symbolic upper bound

parallel: rephrase handling of data lines, symbolic upper bound

Strictly speaking this decoder considers all input signals as optional.
The previous version accepted clock alone. Though reading values from
zero data bits is of limited. Tighten the check for connected inputs.

Inline the declaration of all channels in the decoder boiler plate, the
helper routine was only used in a single spot. Change the order of the
data lines stripe details and the .wait() conditions, improve locality
of assignment and use of related variables.

Don't assume that "all channels but clock" are data lines. Use a
symbolic upper bound for the data lines partition, to prepare the
introduction of a reset/enable signal.