RFC: use a `Signed` wrapper for function returning the sign as an integer

Functions such as Segment::to_running_time_full replicate the C signature for the return type: an integer indicates whether the resulting value must be interpreted as positive or negative. In Rust, alternatives are usually represented using an enum.

Instead of:

    // Declaration
    pub fn to_running_time_full(
        &self,
        position: impl CompatibleFormattedValue<T>,
    ) -> (i32, T::FullRange);

    // Call site
    let end_pts = match segment.to_running_time_full(end_pts_position) {
        (_, None) => // undefined,
        (pts_signum, _) if pts_signum < 0 => // negative,
        (_, Some(pts)) => // positive,
    };

I propose using:

    // Declaration
    pub fn to_running_time_full(
        &self,
        position: impl CompatibleFormattedValue<T>,
    ) -> gst::Signed<T::FullRange>;

    // Call site
    use gst::Signed::*;
    let end_pts = match segment.to_running_time_full(end_pts_position) {
        None => // undefined,
        Negative(_) => // negative,
        Positive(_) => // positive,
    };

We could implement a function Signed::factor that would return None, Some(1i32) or Some(-1i32) for use in multiplications with an integer and of course the is_none, is_positive and is_negative accessors.