[refactor] interpreter: define interface for vfExec
Summary:
Includes comments added by Pieter Wuille.
This is a backport of Core PR16902 [2/3]
https://github.com/bitcoin/bitcoin/pull/16902/commits/d0e8f4d5d8ddaccb37f98b7989fb944081e41ab8
Depends on D8835
Test Plan:
ninja all check-all src/bench/bitcoin-bench -filter=VerifyNestedIfScript
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Subscribers: majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8836