save mesages as a specific class

[topepo]

It would be helpful to class the list with each message. From there, we can add format(), as_tibble(), and other methods to facilitate programming against the elmer api.

[jcheng5]

+1, and maybe classing the list of messages as well. I can imagine wanting to pretty-print subsets of the message history.

[hadley]

Now have the idea of Turns: https://hadley.github.io/elmer/reference/Turn.html