Replace From<u16> by From<u8> for conversions to degrees

[yoanlcq]

The num-traits crate has an interesting way to convert radians to degrees. It boils down to this :

fn to_degrees(self) -> Self {
    self * Self::from(90u8) / (Self::PI / 2.)
}

So far my code has been doing Self::from(360) instead, hence the From<u16> bound.
But now that I've seen this, I would like to use From<u8> instead, which is less restrictive.