Revert stability for Bitv and BitvSet

[nagisa]

The collections were promoted to stable by mistake and do not match RFC 509.
This reverts the stability back to unstable.

[breaking-change] since previously stable API became unstable.

Fixes#21193

r? @gankro

[Gankra]

I'm not great at how the stability attributes work, but I believe it's fine to stabilize the methods, just not the type itself. Basically stating that "I dunno if we're gonna have this type, but if we do, it'll look like this".

[Gankra]

r? @aturon

[aturon]

@gankro

I'm not great at how the stability attributes work, but I believe it's fine to stabilize the methods, just not the type itself. Basically stating that "I dunno if we're gonna have this type, but if we do, it'll look like this".

Yes, exactly. This PR should revert only the types and modules.

[nagisa]

Will do tomorrow.

[nagisa]

Only Bitv and BitvSet structs and collections::{bitv,bitv_set} are unstable now.

[aturon]

@nagisa Thanks! But would you mind also reverting the modules std::collections::bitv and bitv_set to unstable as well?

[nagisa]

Ah, must have forgoten to push that, done.

[steveklabnik]

Doing this manually as part of #21300