Planning for cap-std 4.0.0 #401

To fix #393, #391, #394, and #400, I'm considering doing a major-version bump to cap-std 4.0.0. It's been about 2 years since the last major version bump. If a major bump is inconvenient for anyone, please speak up!

We could avoid it for most crates, though #394 will require a major-version bump for cap-rand, and it's somewhat awkward if the cap-std crates aren't all at the same version; tools like cargo-release take extra configuring. And for #391 we don't strictly guarantee that MSRV bumps get semver major bumps. And for #393 we could just stop releasing cap-async-std without a major bump. And for #400 we could just continue to be incompatible with the current io-lifetimes traits for a while longer yet.

So an across-the-board major bump isn't strictly needed, but would be nice to do if we could.

I'll leave this open a little while for feedback.