Reduce chance of overflow panics on Midpoint Interpolate

[xd009642]

Midpoint interpolate is currently implemented like:

let denom = T::from_u8(2).unwrap();
(lower.unwrap() + higher.unwrap()).mapv_into(|x| x / denom.clone())

This causes overflows most times I use it, whereas implementing like (lower + (higher - lower))/2 would prevent this.

Also, happy to do a PR for this if it would be helpful but I figured it was such a small change it might be quicker if you guys sorted it yourselves 👍

[xd009642]

I see the fix has been merged so I'll close this issue 👍