Release 0.5.0

Full Changelog: v0.4.0...v0.5.0

Massive update!

• Update to rkyv 0.8
• Simplified varint length parsing (no buffer bugs anymore for small packet sizes!)
• Added U8, U16 length codec prefixes
• Updated all dependencies, no bytes-old dependency
• Moved from futures-codec to asynchronous-codec for Encoder/Decoder traits
• Nicer code :)