span: Add BytePtr helper

Summary:
This is a partial backport of core#23438
https://github.com/bitcoin/bitcoin/pull/23438/commits/fa65bbf217b725ada35107b4ad646d250228355c

Depends on D15933

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15934