ENH: align bitSet and FixedList methods names

- bitSet and PackedList fill() as per boost dynamic_bitset.
  Silently deprecate assign(val), which is potentially confusing
  with other forms of assign().

- FixedList fill() as per std::array.
  Add missing assignment from Foam::zero

- minor code simplication in bitSet and PackedList

STYLE: fix typo in comment, qualify Foam::zero