Feature request: Stabilize `NumBufferTrait` to use as a bound

I would like to add the format_into function to my Integer trait. This doesn't seem to be possible because of the T: NumBufferTrait bound in the definition of NumBuffer which requires me to add NumBufferTrait as a super bound to Integer. If NumBufferTrait could be used as a bound, this would allow me to implement this function. A similar issue exists with the to_int_unchecked function and the FloatToInt trait.