Stop using rb_fstring publicly

rb_fstring is a private API, so we should use rb_str_to_interned_str
instead, which is a public API.

ext/-test-/string/fstring.c

@@ -2,20 +2,18 @@
 #include "ruby/encoding.h"
 #include "internal/string.h"
 
-VALUE rb_fstring(VALUE str);
-
 VALUE
 bug_s_fstring(VALUE self, VALUE str)
 {
-    return rb_fstring(str);
+    return rb_str_to_interned_str(str);
 }
 
 VALUE
 bug_s_fstring_fake_str(VALUE self)
 {
     static const char literal[] = "abcdefghijklmnopqrstuvwxyz";
     struct RString fake_str;
-    return rb_fstring(rb_setup_fake_str(&fake_str, literal, sizeof(literal) - 1, 0));
+    return rb_str_to_interned_str(rb_setup_fake_str(&fake_str, literal, sizeof(literal) - 1, 0));
 }
 
 VALUE

spec/ruby/optional/capi/ext/string_spec.c

@@ -51,10 +51,8 @@ VALUE string_spec_rb_str_set_len_RSTRING_LEN(VALUE self, VALUE str, VALUE len) {
   return INT2FIX(RSTRING_LEN(str));
 }
 
-VALUE rb_fstring(VALUE str); /* internal.h, used in ripper */
-
 VALUE string_spec_rb_str_fstring(VALUE self, VALUE str) {
-  return rb_fstring(str);
+  return rb_str_to_interned_str(str);
 }
 
 VALUE string_spec_rb_str_buf_new(VALUE self, VALUE len, VALUE str) {

string.c

@@ -398,7 +398,6 @@ fstr_update_callback(st_data_t *key, st_data_t *value, st_data_t data, int exist
     }
 }
 
-RUBY_FUNC_EXPORTED
 VALUE
 rb_fstring(VALUE str)
 {