Make add, length into fully overloadable operators

- add and length are each in a module of their own, providing the
  overload infrastructure.
- value_seq and value_str are each in a module of their own, together
  with relevant overloads.

- init.cc was introduce to handle initialization of all the builtins
  in the right order.  It is forseeable that there will be a separate
  dwinit for the Dwarf stuff.  It will be possible to omit this from
  linking to get Dwarf-unaware builds--such as test-parser.cc, or
  possibly light-weight runners for testing Dwarf-unrelated stuff.