feat: RefMut::into_mut_unchecked

[aatifsyed]

My usecase is

pub struct Yak {
    id: YakId,
    pub name: String,
}
impl Yak {
    fn id(&self) -> YakId;
}
struct YakMgr(iddqd::IdHashMap<Yak>);
impl YakMgr {
    fn get_mut(&mut self, id: YakId) -> &mut Yak;
}

Users have no access to id

[aatifsyed]

Hi @sunshowers, any enthusiasm for this PR? :)

[sunshowers]

Thanks. I'm generally open to this, but ideally I'd not put this on RefMut because (for IdOrdMap) that requires that the key type implements Hash. Instead I'm wondering if it makes sense to have get_mut_unchecked (and so on) on the map types directly. I'm not sure, that seems like it bloats up the API a bunch.