Integrate valfile into dwgrep, and new approach to yielders

- Yielders are no more, instead each operator knows about its
  upstream, fetches data by simply calling upstream's next, and
  doesn't clone the stack on every occasion.  This is a very rough
  sketch that I just wanted commited, it's not particularly complete.
  Empty query works, @at works as well, no predicates though.

- Initial numbers show almost 3x performance boost for empty query
  (i.e. just go through all the DIE's and write their offsets).
  That's about 2x slow-down vs. C++ code doing the same thing using
  all_dies_iterator (but it's so basically because I/O is so slow, the
  actual dwgrep overhead will be obviously worse).
Rewrite child, offset for the new approach
Drop unused value.cc/.hh
Support attribute values, ?tag, ?@at, ?eq, !, and constant literals
op_const should hold a constant object, not a general value
Support ?lt, ?gt, ?le, ?ge, ?root
Implement ?{}
Drop the oldsize/newsize dichotomy

- Let's keep things simple until this is proven to be a performance problem
Expose Dwarf-related constant domains
Simplify writing DIE-specific operators
Implement name, tag, form, attribute, and string literals
Implement value; and most constant values now have domain assigned

- ... meaning they are shown symbolically, rather than with their
  actual underlying value.
s/emt/elt/g
Implement formatted strings, dup and drop