Merge changes I46bf0c83,I5f639a94,Ib4ff14eb,I59c1d4c4,Ib9147d48

* changes:
  pdl: push length checks down to ‘Chunk’
  pdl: add new ‘Chunk’ struct
  pdl: move ‘generate_field_getter’ to ‘Field’
  pdl: move ‘generate_field’ to ‘Field’
  pdl: create ‘Field’ enum to hold field logic