`midpoint` function

[tyilo]

I think it would be very useful to have the midpoint function in num_traits for implementing binary search.

I'm currently using:

pub trait Midpoint: Num {
    fn midpoint(&self, rhs: &Self) -> Self;
}

impl<T> Midpoint for T
where
    T: Num + Shr<u8, Output = T>,
    for<'a> &'a T: BitXor<&'a T, Output = T> + BitAnd<&'a T, Output = T>,
{
    fn midpoint(&self, rhs: &Self) -> Self {
        ((self ^ rhs) >> 1u8) + (self & rhs)
    }
}

[tyilo]

This already exists as Average in the num-integer crate.