refactor: Treat CDataStream bytes as uint8_t

Summary:
Using uint8_t for raw bytes has a style benefit:

• The signedness is clear from reading the code, as it does not depend on the architecture

Other clean-ups in this pull include:

• Remove unused methods
• Constructor is simplified with Span
• Remove Init() member in favor of C++11 member initialization

Notes:

• the += operatior is not unused in our codebase
• the seeder code needs minor adjustments to use the Span constructor

This is a backport of core#20464

Test Plan: ninja all check-all bench-bitcoin bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14689