More Span simplifications

Summary:
Based on suggestions by MarcoFalke <falke.marco@gmail.com>

This concludes backport of core#23413
https://github.com/bitcoin/bitcoin/pull/23413/commits/11daf6ceb1d9ea1f8d638b123eecfe39d162a7c3

Depends on D14768

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14769