span: Add std::byte helpers

Summary:
Also, add Span<std::byte> interface to strencondings.

This concludes backport of core#23451
https://github.com/bitcoin/bitcoin/pull/23451/commits/faa3ec2304051be7cfbe301cfbfbda3faf7514fc

Depends on D15858

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15859