Create internal/set.h for set.c function definitions

Merged: https://github.com/ruby/ruby/pull/18776

array.c

@@ -23,6 +23,7 @@
 #include "internal/object.h"
 #include "internal/proc.h"
 #include "internal/rational.h"
+#include "internal/set.h"
 #include "internal/string.h"
 #include "internal/vm.h"
 #include "probes.h"
@@ -6751,8 +6752,6 @@ rb_ary_count(int argc, VALUE *argv, VALUE ary)
     return LONG2NUM(n);
 }
 
-VALUE rb_ident_set_new(void);
-
 static VALUE
 flatten(VALUE ary, int level)
 {