Move initialization of words for comparison predicates to init.cc
Move initialization of stack shuffling words to init.cc
Expose initialization of Dwarf-related words in dwgrep_init_dw
Introduce valfile::pop_as

- Since overload dispatch takes care of type checking for us, we can
  avoid the overhead (sans an inevitable assert) in the operator code.
Move value to builtins
Move value_attr, value_die into value-dw.cc/.hh
Fix parser test suite

- This was broken mostly because it would bring in all the Dwarf
  stuff, libdw etc. in order to check a parser.  Now that we have
  properly separated common builtins from Dwarf-related ones, we can
  link the parser test with dwgrep core only.