test: Fix outstanding -Wsign-compare errors
refactor: Rework asmap Interpret to avoid ptrdiff_t
This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/18216 | PR18216]]
It also includes a change in asmap.cpp that was missed in D9055 ([[https://github.com/bitcoin/bitcoin/pull/18512 | PR18512]])