style: rename internal "stype" as "union_members" for clarity

"stype" is quite unclear, and it also collides with the former %define
variable that had the same name (replaced by api.value.type).

* src/parse-gram.y (stype): Rename as...
(union_members): this.
* data/bison.m4: Adjust.
(b4_user_stype): Rename as...
(b4_user_union_members): this.
* data/c++.m4, data/c.m4: Adjust.
* src/parse-gram.c: regen.

data/bison.m4

@@ -577,14 +577,14 @@ m4_define([b4_define_user_code],
 # b4_user_initial_action
 # b4_user_post_prologue
 # b4_user_pre_prologue
-# b4_user_stype
+# b4_user_union_members
 # ----------------------
 # Macros that issue user code, ending with synclines.
 b4_define_user_code([actions])
 b4_define_user_code([initial_action])
 b4_define_user_code([post_prologue])
 b4_define_user_code([pre_prologue])
-b4_define_user_code([stype])
+b4_define_user_code([union_members])
 
 
 # b4_check_user_names(WHAT, USER-LIST, BISON-NAMESPACE)

data/c++.m4

@@ -117,10 +117,10 @@ m4_define([b4_token_enums],
 # Declare semantic_type.
 m4_define([b4_semantic_type_declare],
 [    /// Symbol semantic values.
-m4_ifdef([b4_stype],
+m4_ifdef([b4_union_members],
 [    union semantic_type
     {
-b4_user_stype
+b4_user_union_members
     };],
 [m4_if(b4_tag_seen_flag, 0,
 [[    typedef int semantic_type;]],

data/c.m4

@@ -501,11 +501,11 @@ b4_locations_if([, yylocationp])[]b4_user_args[);
 m4_define([b4_value_type_define],
 [[/* Value type.  */
 #if ! defined ]b4_api_PREFIX[STYPE && ! defined ]b4_api_PREFIX[STYPE_IS_DECLARED
-]m4_ifdef([b4_stype],
+]m4_ifdef([b4_union_members],
 [[typedef union ]b4_union_name[ ]b4_api_PREFIX[STYPE;
 union ]b4_union_name[
 {
-]b4_user_stype[
+]b4_user_union_members[
 };
 # define ]b4_api_PREFIX[STYPE_IS_TRIVIAL 1]],
 [m4_if(b4_tag_seen_flag, 0,