Make script/standard's BaseHash Span-convertible

Summary:
Partial backport (4/8) of core PR19326:
https://github.com/bitcoin/bitcoin/pull/19326/commits/2a2182c387f607cd8284f33890bd285a81077b7f

Depends on D9149.

Test Plan:

ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9150