Rename master to main

[asmeurer]

I think we never did this because it might break upstream users. We should check if that would cause any problems (maybe with a github code search).

[ev-br]

IMO it's a no-brainer. No matter whether it's "main" or "master", the cognitive load of a single change is trivial; what gets annoying is having to switch between one and the other often (as in, -compat has main, -strict has main, and -tests is "master").

Whether this is a problem for upstream, I seriously cannot think of a scenario where something depends on the name of a default branch in a hard-to-recover-able way?