Re-use AllocVec

[BertrandBev]

let out = serialize_with_flavor(&message, AllocVec::new());

That method returns the inner Vec.
It should be great to be able to re-use an existing AllocVec to avoid repeated allocations when serializing a bunch of types. All we'd need is a

impl From<Vec<u8>> for AllocVec {
        fn from(vec: Vec<u8>) -> Self {
            Self(vec)
        }
    }

Would that make sense ?