Replace READWRITEAS macro with AsBase wrapping function

Summary:
Co-authored-by: Pieter Wuille <pieter@wuille.net>

This is a partial backport of core#25284
https://github.com/bitcoin/bitcoin/pull/25284/commits/aaaa3fa9477eef9ea72e4a501d130c57b47b470a

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19054