Call it 1.0.0

I'm suggesting to call this 1.0.0, even though it's very new. You may disagree; if you have qualms, I'm happy to wait.

Things to consider:

• Are we happy with the API? I think almost everything about it is obvious except try_push_or_discard and its error type, and try_push.

• Are we happy with its level of software integrity? I added a bunch of tests, which are run with miri, in CI. I think they exercise most of the corner cases, but I haven't done any formal coverage testing.

• Is there any other reason we wouldn't want to call this 1.0 now?

If you would rather not declare it 1.x right awy (which as I say would be a reasonable point of view) I'd still like to remove the warning from the README, since I think I've done most of the things I meant when I wrote it.

(I won't make an actual release without also having !14 (merged) merged)