serialize: Serialization support for big-endian 32-bit ints.

Summary:
backport of a small commit from BIP157 PR 14121

2ad2338ef serialize: Serialization support for big-endian 32-bit ints.

Test Plan: no tests included

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Maniphest Tasks: T589

Differential Revision: https://reviews.bitcoinabc.org/D2873