Add proposed/accepted status to design docs

[eemeli]

We should track and signal the status of a design document, so that we can iterate on them across multiple PRs. To keep it simple, we probably don't need more than two statuses to start with, "proposed" and "accepted".

CC @ryzokuken, who also has a design doc PR in flight.

[aphillips]

But otherwise ready...

[eemeli]

Requesting fast-tracking.

[stasm]

In my mind, a design doc would only be merged into main when it's accepted. Furthermore, I think that splitting a design doc into multiple PRs may again make it more difficult to track the discussion around it. Instead, I think we should encourage long-lived PRs and multiple rounds of reviews.

[eemeli]

@stasm Could you review #458, as that's effectively the prototypical reason for this change?

It's currently at 35 comments in 13 threads of review comments, most of which are hidden as resolved. If we don't make this change, we won't be able to merge the PR until all expression attribute design questions are answered, including e.g. whether it's the proper vessel for communicating "open" vs. "closed" concepts which have not yet been mentioned at all.