Allow Class#allocate to be prohibited

Calling `Class#allocate` with `UnboundMethod#bind_call` usually succeeds without problems.
If this behavior does not make us happy, we can now prohibit it with `MRB_SET_INSTANCE_TT(klass, MRB_TT_UNDEF)`.

At the same time, it applies to the `Binding`, `Complex`, `Data`, `Float`, `Integer`, `Method`, `Rational` and `UnboundMethod` classes.

src/class.c

@@ -1945,6 +1945,9 @@ mrb_instance_alloc(mrb_state *mrb, mrb_value cv)
     ttype = MRB_TT_OBJECT;
   }
   if (ttype <= MRB_TT_CPTR) {
+    if (ttype == MRB_TT_UNDEF) {
+      mrb_raisef(mrb, E_TYPE_ERROR, "allocator undefined for %v", cv);
+    }
     mrb_raisef(mrb, E_TYPE_ERROR, "can't create instance of %v", cv);
   }
   o = (struct RObject*)mrb_obj_alloc(mrb, ttype, c);