add missing provenance APIs on NonNull

[RalfJung]

This adds some provenance APIs that exist on raw pointers but have been forgotten on NonNull:

impl<T> NonNull<T> {
    pub const fn without_provenance(addr: NonZero<usize>) -> Self;
    pub fn from_exposed_provenance(addr: NonZero<usize>) -> Self;
}
impl<T: ?Sized> NonNull<T> {
    pub fn expose_provenance(self) -> NonZero<usize>;
}

rust-lang/libs-team#518 is the ACP for the two exposed provenance ones; I forgot to include without_provenance there but I hope that, too, is uncontroversial (and anyway this PR only adds things unstably). Cc @rust-lang/libs-api

Tracking issue: #135243