[LINTER] Check for missing explicit

Summary:
Fix the remaining missing explicit occurrences and add a linter to
prevent introducing new ones.
The linter uses cppcheck, which is natively supported by arcanist.
However we need some more output filtering so I redefined another linter
class inpired by their work.

Test Plan:

arc lint --everything
ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6549