Rename `master` branch `main`

[toots]

Is there any opposition to this?

[smuenzel]

https://github.com/ocaml/ocaml/ uses 'trunk' as the name for this branch. I don't know if we want to be consistent

[Octachron]

https://github.com/ocaml/ocaml uses trunk because the repository is tracking Caml history since the conversion from cvs to svn in 1995. I don't think that it is necessary to use that naming scheme without the corresponding historical context.

[toastal]

trunk is still a good name if trunk-based development is used (especially with patches or stack-based diffs).