Add abbreviation units

- wabbrev now iterates abbreviation units instead of individual
  abbreviations.  "elem" and "offset" are words applicable to
  abbreviation units.
Implement T_CU

- A value for holding compilation unit.  Applicable words: offset,
  abbrev, version, root.

- unit::T_DIE became -> T_CU.

- unit::T_DWARF was added with prototype ->* T_CU.
Change wabbrev to abbrev