Expand Number functionality (minor breaking change)

[danlehmann]

Number currently puts few restrictions on the underlying type:

pub trait Number: Sized {
    type UnderlyingType: Debug
        + From<u8>
        + TryFrom<u16>
        + TryFrom<u32>
        + TryFrom<u64>
        + TryFrom<u128>;
  ..
}

But maybe it should do more: Number could require that UnderlyingType supports Add, Sub etc so that it itself can do that, too. That would make it possible to calculate with Numbers.

[dzmitry-lahoda]

is it used anywhere? if not, no need.

and, e.g. one will impl log or pow, he can just restrict UT as needed in place.

so just suggestion to close item.