Change F_ATVAL to F_ATTR_NAMED

- F_ATVAL is really F_ATTR_NAMED followed by a F_VALUE.  Otherwise the
  functionality of F_VALUE is duplicated in F_ATVAL.
Look for type in enumeration_type to decode const_value of enumerator

- Unfortunatelly there are no tests for this ATM, as GCC doesn't
  produce DW_AT_type at DW_TAG_enumeration_type, instead encoding
  enumerators as DW_FORM_udata or DW_FORM_sdata to denote signedness.
Don't format all DIE attribute values as hex
Add tests/ to make check
Support DW_AT_stmt_list and DW_FORM_sec_offset in atval

- op_f_attr still needs to handle these specially for full support,
  where accessing an attribute yields a number of nodes that are
  logically connected to this attribute.  The support in atval is
  meant for value_attr::show.