New cpp11::messages with opt-in fmt formatting

[sbearrows]

Fixes #208

Let me know what you think!

[sbearrows]

I had a bunch of merge conflicts so I'm not surprised that format_check is failing. I'm gonna wait to see if the other checks pass and then I'l fix it!

Well that was a fail.

[jimhester]

could you rebase this branch on the current master? I can help you do it in our meeting if you need to, but assuming r-lib/master is the 'upstream' remote you would do something like.

git checkout feature-message_fmt
git fetch upstream
git rebase upstream/master

I think your branch is based on your forks master branch, which is very out of date at this point, so we are getting a bunch of extra commits in the PR.

[sbearrows]

Yes, please and thank you!

[jimhester]