Function ra_ap_stdx::partition_point

pub fn partition_point<T, P>(slice: &[T], mut pred: P) -> usize where
    P: FnMut(&T) -> bool, 

Returns idx such that:

    ∀ x in slice[..idx]:  pred(x)
 && ∀ x in slice[idx..]: !pred(x)

https://github.com/rust-lang/rust/issues/73831