Builtins expose more information about their behavior

- In particular, they can new return a mapping from stack profiles to
  yielding behavior and what they push to TOS.  The common overload
  base classes build this information from what the subclasses tell
  them.  All operators were updated to declare their return value.
  Some were updated to declare they always yield exactly once (though
  more are likely to come).

- Another addition is that every builtin can now return a docstring.

- This will all be used for autogenerating reference documentation for
  Zwerg words.  A very preliminary sketch of this tool has been added
  is dwgrep-gendoc.cc.