glr2.cc: don't use YYSTYPE/YYLTYPE at all

* data/skeletons/glr2.cc: Define value_type and location_type where
needed, and use them only.
(yyuserMerge): Make it a member function of the glr_state class.

TODO

@@ -211,10 +211,6 @@ Some occurrences were fixed, but now some have improper names:
 
 yycharp is no longer a Pointer.  And yystackp should probably also be a reference.
 
-*** Use proper type names
-Don't use YYSTYPE and YYLTYPE but parser::semantic_type and
-parser::location_type.  Undefine YYSTYPE and YYLTYPE.
-
 *** parse.assert
 Currently all the assertions are enabled.  Once we are confident in glr2.cc,
 let parse.assert use the same approach as in lalr1.cc.