roots of unity

Stream: PR reviews

[Johan Commelin]

Sorry for the PR mess that I created. I think it should be stable now.
By now there should be a rather fleshed out API for (primitive) roots of unity

#4330 -- complex roots of unity [depends on below]
#4260 -- roots of unity (general integral domain) [deps on below]
#4329 -- rename is_unit_pow to is_unit.pow. easy
#4322 -- generic preps for #4260