adding add/sub method for Time struct

[alexandru0-dev]

Adding add and sub methods to Time, would be useful for calculating seek times for playbacks

My temporary solution:

// symphonia-core/src/units.rs

impl Time {
// ...

    pub fn add(self, other: Self) -> Self {
        Time {
            seconds: self.seconds + other.seconds,
            frac: self.frac + other.frac
        }
    }

    pub fn sub(self, other: Self) -> Self {
        // check for underflow
        let seconds = self.seconds.checked_sub(other.seconds).unwrap_or(0);
        let mut frac = self.frac - other.frac;
        if frac < 0.0 {
            frac = 0.0
        }
        Time {
            seconds,
            frac
        }
    }

... 
}

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