[backport#13769] Mark single-argument constructors "explicit"
Summary:
https://github.com/bitcoin/bitcoin/pull/13769/commits/1ac3c983bfe06f75542e4f4e30142952802a46d8
This is a backport of Core PR13769
Test Plan:
ninja check-all
Reviewers: #bitcoin_abc, nakihito
Reviewed By: nakihito
Differential Revision: https://reviews.bitcoinabc.org/D6072