A more compact way to write op_root_die::operate
Replace T_ARANGE with T_ASET

- This changes the simple low/high pair that was address range with a
  more complex notion of address sets.  Unlike address ranges, address
  sets can contain disjoint sets of addresses, and thus we can allow
  general arithmetic over address sets.

- The code of coverage.cc/.hh is taken from dwarflint and adapted in
  minor ways.  The interface is fairly awkward mostly because this all
  used to be C at some point.
Updates to README