[Chronik] Add Group::ser_member, remove Into<Vec<u8>> bound on Group::Member

Summary: This makes the code cleaner (no newtyping necessary to get custom serialization), and more flexible (groups can use &self) when serializing.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13539