crypto: refactor ChaCha20 classes to use Span<std::byte> interface

Summary:
This is a partial backport of core#28100
https://github.com/bitcoin/bitcoin/pull/28100/commits/3da636e08b781fa2f7c1c23bb937015185732a75

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19034