Implement more builtins

- low, high for accessing low and high ends of an address range

- arange for building an address range from a pair of ends

- address is now applicable to attributes with address form, or
  high_pc or entry_pc of constant form--those are translated to
  addresses.
Use address domain for value of address attributes
Drop an item in README

- Attributes should probably interpret rather less than more, so that
  dwgrep can be used to this sort of querying.  If the interpreted
  behavior is necessary, one can use attribute ?AT_foo address, which
  is verbose, but gets the job done.  Chances are "address", "high" and
  "low" over T_DIE will take care of typical uses of this.
Implement low, high for T_DIE