gstreamer: Implement `From<T> for Signed<T>`

This makes usage simpler in a few places.

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CC @fengalin any reason not to?

requested review from @fengalin

I think the only reason I didn't implement it yet was that into_positive and into_negative seemed more explicit to me. But I guess constructing from an unsigned integer is pretty explicit too:

fn foo(signed: impl Into<Signed<u64>>) {
    [...]
}

foo(0u64);

So yes, it seems useful to me :)

approved this merge request

assigned to @gstreamer-merge-bot

mentioned in commit slomo/gstreamer-rs@774fafd9

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• 774fafd9 - gstreamer: Implement `From<T> for Signed<T>`

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