refactor: Switch serialize to uint8_t

Summary:
Replace char -> uint8_t in serialization where a sign doesn't make sense (char might be signed/unsigned).

This is a backport of core#21969

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15929