fix OSX build failure related to Span deduction guide in macro

Summary:
It seems that some compilers don't support user-defined deduction guides (D14768) wrapped in macros.
So create the Span before using it in READWRITE.

Test Plan:
ninja all check-all
build osx

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14775