Fix compilation with alloc and experimental-derive (#145)

```sh
RUSTFLAGS='--check-cfg=cfg(doc_cfg)' cargo check --features=alloc,experimental-derive
```

src/max_size.rs

@@ -2,7 +2,7 @@
 extern crate alloc;
 
 #[cfg(feature = "alloc")]
-use alloc::{rc::Rc, sync::Arc};
+use alloc::{boxed::Box, rc::Rc, sync::Arc};
 
 use crate::varint::varint_max;
 use core::{