constify pointer from RARRAY_PTR to detect potential write barrier bugs.

if you see compiler errors due to this commit, you'd better to use array-modifying
functions, e.g. mrb_ary_set() or mrb_ary_push(), otherwise you might see nasty
GC bugs in the future.  if you are sure what you are doing, replace `RARRAY_PTR(ary)`
by `mrb_ary_ptr(ary)->ptr`. but be warned.

src/array.c

@@ -574,7 +574,7 @@ mrb_ary_splice(mrb_state *mrb, mrb_value ary, mrb_int head, mrb_int len, mrb_val
 {
   struct RArray *a = mrb_ary_ptr(ary);
   mrb_int tail, size;
-  mrb_value *argv;
+  const mrb_value *argv;
   mrb_int i, argc;
 
   ary_modify(mrb, a);