Bitv/BitvSet's #[stable] mismatches collections reform RFC

[huonw]

@nikomatsakis noticed that Bitv and BitvSet are marked #[stable] but collections reform 2 says:

Unstable: Bitv, BitvSet, VecMap

(Maybe there was further discussion not in the RFC, maybe not.)

cc @gankro, @aturon

[Gankra]

Definitely an error in my stabilization pass.