dwarf: Implement it.

Better support for DWARF. What works now: DWARF 2 debug_info, debug_abbrev, debug_lines parsing. gcc always generates debug_lines in version 2 format, so it should work well for most of the binaries.

Testing done: Compared output of 'id' command with objdump -g on r2 binary itself (compiled with -gdwarf-2 flag). DWARF {3,4} are to follow.