Fix super from aliased methods to work correctly; fix #4718

We needed to preserve the original method name somewhere. We kept it in
the `env` structure pointed from aliased methods. #1457 and #1531 tried
to address this issue. But this patch is more memory efficient.

Limitation: this fix does not support `super` from methods defined by
`define_method`. This limitation may be addressed in the future, but
it's low priority.

src/proc.c

@@ -77,6 +77,9 @@ closure_setup(mrb_state *mrb, struct RProc *p)
       e->c = tc;
       mrb_field_write_barrier(mrb, (struct RBasic*)e, (struct RBasic*)tc);
     }
+    if (MRB_PROC_ENV_P(up) && MRB_PROC_ENV(up)->cxt == NULL) {
+      e->mid = MRB_PROC_ENV(up)->mid;
+    }
   }
   if (e) {
     p->e.env = e;