actorlib/structuring: add Chunkerize/Unchunkerize

in case where data widths are converted and where the layout
does have only field, we can not use Cast, ex:
  for [("d", 32), ("be", 4)]
  we want to have 4 chunks of  [("d", 8), ("be", 1)]
using Cast will not split data correctly.
actorlib: assert sop/eop only when stb is asserted (not mandatory, but cleaner)