parallel: address minor style nits, no change in behaviour

parallel: address minor style nits, no change in behaviour

Remove the redundant .itemcount variable, which exactly corresponds to
the length of the .items array.

Arrange retrieval of options and their evaluation closer to each other
for improved readability.

Use common logic to construct "words" from several multi-bit "items".
Arrange for endianess support by optionally reversing the array before
traversal.