feat(packet): implement From<Fulfill/Reject> for bytes05::BytesMut

ILP-Packet is built using Bytes 0.4. The Futures 0.3 ecosystem's HTTP crates use Bytes 0.5.
Porting this crate to use Bytes 0.5 is non-trivial due to significant breaking changes in the
Bytes API:

tokio-rs/bytes#350
tokio-rs/bytes#288

Author: gakonst