BoxedUint operand flexibility

Because the operations are generally not `const`, a number of `BoxedUint` methods could accept `&impl Unsigned`. This would make methods like `BoxedUint::mul` more powerful, getting rid of the need for `mul_unsigned`, and make it less awkward to mix `Uint` into `BoxedUint` usage. (edited to remove suggestion for `AsRef<UintRef>` as that would include signed types)