Add compile time verification of assumptions we're currently making implicitly/tacitly

Summary:
Add compile time verification of assumptions we're currently making implicitly/tacitly.

Backport of Bitcoin Core PR15391
https://github.com/bitcoin/bitcoin/pull/15391

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4043