`SerializedTimeSpan` struct

[Huchell]

Implement a SerializedTimeSpan struct which can be used to abstract the timespan of a serialized field.

It should be convertible to System.TimeSpan implicitly, and can be explicting made from System.TimeSpan so the following is true

public class Foo
{
    [SerializeField]
    private SerializedTimeSpan duration;

    public void Test()
    {
        TimeSpan var1 = duration;
        SerializedTimeSpan var2 = (SerializedTimeSpan)var1;
    }
}