Add PrimitiveNumber methods for as type-casting

[cuviper]

trait PrimitiveNumber {
    // ...

    /// Creates a number using a type cast, `value as Self`.
    fn as_from<T>(value: T) -> Self
    where
        Self: PrimitiveNumberAs<T>;

    /// Returns this number converted with a type cast, `self as T`.
    fn as_to<T>(self) -> T
    where
        Self: PrimitiveNumberAs<T>;
}

The helper PrimitiveNumberAs is also added as a supertrait for all
concrete primitive numbers, so the methods can be used without any
additional bounds if you're converting to/from specific types.