Merge #12250: Make CKey::Load references const

Summary:
04ededf Make CKey::Load references const (Russell Yanofsky)

Pull request description:

No change in behavior, this just prevents CKey::Load arguments from looking
like outputs.

Tree-SHA512: 6d93bce109318e88ddd5c21ad626571344707ae0e6d46e898c76fd95a7afd1c32202a6b3dfab47d6a787c84dfcbb35343cdec898bcf8f668574aa224f2eed977

Backport of Core PR12250

https://github.com/bitcoin/bitcoin/pull/12250/files

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3380