Check assumptions on integer implementation at compile time

Summary:

• Add support for (signed) __int128
• Compile-time check assumptions on integer types

This is a backport of secp256k1 PR798

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7632