Renaming master -> main

[jneen]

This has been waiting for quite some time, and I think it's about time we tear off this band-aid. Github these days has features that makes this fairly painless to do.

I'll go through with this in the next week or so absent any objections, @tancnle what do you think?

[tancnle]

@jneen Sounds good to me 👍🏼