Function par_mergesort 

pub(super) fn par_mergesort<T, F>(v: &mut [T], is_less: F)
where
    T: Send,
    F: Fn(&T, &T) -> bool + Sync,

Sorts v using merge sort in parallel.

The algorithm is stable, allocates memory, and O(n log n) worst-case. The allocated temporary buffer is of the same length as is v.