Use rb_eArgError directly.

It's part of the public API, so little point keeping another
private reference to it.

Additionally when storing an object in a global variable you MUST
register that variable to the gc with `rb_global_variable(&cArgumentError)`
and that *before* you store anything in it, so that the GC knows to
update your global variable if it moves that object.

In practice `ArgumentError` is pinned for now, so it would have been
OK, but it might not be pinned forever.

ext/iou/ring.c

@@ -4,7 +4,6 @@
 
 VALUE mIOU;
 VALUE cRing;
-VALUE cArgumentError;
 
 VALUE SYM_accept;
 VALUE SYM_block;
@@ -151,15 +150,15 @@ static inline struct io_uring_sqe *get_sqe(IOU_t *iou) {
 
 static inline void get_required_kwargs(VALUE spec, VALUE *values, int argc, ...) {
   if (TYPE(spec) != T_HASH)
-    rb_raise(cArgumentError, "Expected keyword arguments");
+    rb_raise(rb_eArgError, "Expected keyword arguments");
 
   va_list ptr;
   va_start(ptr, argc);
   for (int i = 0; i < argc; i++) {
     VALUE k = va_arg(ptr, VALUE);
     VALUE v = rb_hash_aref(spec, k);
     if (NIL_P(v))
-      rb_raise(cArgumentError, "Missing %"PRIsVALUE" value", k);
+      rb_raise(rb_eArgError, "Missing %"PRIsVALUE" value", k);
     values[i] = v;
   }
   va_end(ptr);
@@ -294,13 +293,13 @@ VALUE IOU_prep_cancel(VALUE self, VALUE spec) {
     return prep_cancel_id(iou, NUM2UINT(spec));
 
   if (TYPE(spec) != T_HASH)
-    rb_raise(cArgumentError, "Expected operation id or keyword arguments");
+    rb_raise(rb_eArgError, "Expected operation id or keyword arguments");
 
   VALUE id = rb_hash_aref(spec, SYM_id);
   if (!NIL_P(id))
     return prep_cancel_id(iou, NUM2UINT(id));
 
-  rb_raise(cArgumentError, "Missing operation id");
+  rb_raise(rb_eArgError, "Missing operation id");
 }
 
 VALUE IOU_prep_close(VALUE self, VALUE spec) {
@@ -719,8 +718,6 @@ void Init_IOU(void) {
   rb_define_method(cRing, "process_completions", IOU_process_completions, -1);
   rb_define_method(cRing, "process_completions_loop", IOU_process_completions_loop, 0);
 
-  cArgumentError = rb_const_get(rb_cObject, rb_intern("ArgumentError"));
-
   SYM_accept        = MAKE_SYM("accept");
   SYM_block         = MAKE_SYM("block");
   SYM_buffer        = MAKE_SYM("buffer");