Rename branch to `main`

[mwtoews]

This task is to rename this project's main branch to main. This has just been done with the GitHub UI, but will require contributors to update their local clones:

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

Other individual forks (124 of them as this moment) may also want to rename their forked repo, which can be done with the GitHub UI.

[hobu]

This is now complete AFAICT. Please rename your local branches if you are tracking master.