Zero copy byte deserialization

[iblnkn]

We're using this crate to deserialize ROS2 sensor messages (images, point clouds) and needed efficient handling of large byte payloads.

Changes:

1. Implements the bytes path correctly so byte payloads can be
   serialized and deserialized as contiguous blocks instead of
   one byte at a time through the seq path.

2. Fixes the deserializer's lifetime constraints so borrowed slices
   can reference the input buffer directly, enabling zero-copy
   deserialization.

Added tests

• Round-trip tests for owned bytes, empty bytes, borrowed bytes, and
  bytes embedded in a struct
• Cross-path test: data serialized as Vec<u8> can be deserialized
  through the bytes path
• Regression test confirming Vec<u8> without serde_bytes still works