move mrb_shared_string definition to src/string.c

src/string.c

@@ -23,6 +23,12 @@
 
 const char mrb_digitmap[] = "0123456789abcdefghijklmnopqrstuvwxyz";
 
+typedef struct mrb_shared_string {
+  int refcnt;
+  char *ptr;
+  mrb_int len;
+} mrb_shared_string;
+
 static mrb_value str_replace(mrb_state *mrb, struct RString *s1, struct RString *s2);
 static mrb_value mrb_str_subseq(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len);