message: mock: implement Readable for Value
diff: +use chain_core::mempack::{ReadBuf, ReadError, Readable};
  use chain_core::property;
  use std::ops;
  @@ -62,6 +63,11 @@ impl property::Deserialize for Value {
  codec.get_u64().map(Value)
  }
  }
  +impl Readable for Value {
  + fn read<'a>(buf: &mut ReadBuf<'a>) -> Result<Self, ReadError> {
  + buf.get_u64().map(Value)
  + }
  +}
  impl property::Serialize for Value {
  type Error = std::io::Error;
  