[cpp] move to C++17

Summary:
Debian buster ships with gcc 7/8 which is all green according to:
https://gcc.gnu.org/projects/cxx-status.html

Depends on D8492 and D8495

edit: backported some changes from Core PR18591 regarding implicit conversion between Span types

Test Plan:

ninja all check
test_runner.py --extended

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8493