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