Let symbols have a location.

* src/symtab.h, src/symtab.c (symbol_t): Location is a new member.
(getsym): Adjust.
Adjust all callers.
* src/complain.h, src/complain.c (complain_at, fatal_at, warn_at):
Use location_t, not int.
* src/symtab.c (symbol_check_defined): Take advantage of the
location.
* tests/regression.at (Invalid inputs): Adjust.

src/parse-gram.y

@@ -336,7 +336,7 @@ rhs:
 symbol:
   ID              { $$ = $1; }
 | string_as_id    { $$ = $1; }
-| CHARACTER       { $$ = getsym ($1); }
+| CHARACTER       { $$ = getsym ($1, @1); }
 ;
 
 action:
@@ -348,7 +348,7 @@ action:
 string_as_id:
   STRING
     {
-      $$ = getsym ($1);
+      $$ = getsym ($1, @1);
       symbol_class_set ($$, token_sym);
     }
 ;