random: write rand256() and rand160() in function of fillrand()

Summary:
This is a partial backport of core#29625
https://github.com/bitcoin/bitcoin/pull/29625/commits/493a2e024e845e623e202e3eefe1cc2010e9b514

note the rand160() was introduced in D8496

Depends on D19040

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19041