Fix build on Crystal 1.18.0-dev

https://github.com/iv-org/protodec/actions/runs/17175002870/job/48729686142

```
 933 | def read_bytes(type : T.class, format : IO::ByteFormat = IO::ByteFormat::SystemEndian) : T forall T
                                                                                                ^
Error: Error: method IO::Memory#read_bytes must return Protodec::VarLong but it is returning Int64
```