util: Add Assert identity function
Summary:
The utility function is primarily useful to dereference pointer types, which are known to be not null at that time. For example, the ArgsManager is known to exist when the wallets are started: https://github.com/bitcoin/bitcoin/pull/18923/files#diff-fdb2a1a1d8bc790fcddeb6cf5a42ac55R503 . Instead of silently relying on that assumption, Assert can be used to abort the program and avoid UB should the assumption ever be violated.
Backport of core PR19277.
Depends on D8565.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8566