crypto: make ChaCha20::SetKey wipe buffer
Summary:
This concludes backport of core#28100
https://github.com/bitcoin/bitcoin/pull/28100/commits/57cc136282c38825e97bbf85728df4bdf1ccc648
Depends on D19038
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D19039