Declare single-argument (non-converting) constructors "explicit"
Summary:
In order to avoid unintended implicit conversions.
Backport of core PR10969
Depends on D2205, D2206 and D2212
Test Plan:
make check
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, deadalnix, jasonbcox
Subscribers: jasonbcox, teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2207