Allow to add and subtract durations from `Utc`

[ChristopherRabotin]

This should return another Utc. This will be needed to support loops over time, such as:

let mut current = Utc::new(/*...*/);
let end =  Utc::new(/*...*/);
while current < end {
/* do stuff */
    current += std::time::Duration(1, 0) // Or current += 1*SECOND
}

Eventually also include in this PR some short cuts such as:

hifitime::constants::{DAY, HOUR, MINUTE, SECOND, MICROSECOND, NANOSECOND};

Note that these short cut will need to support multiplications with floats and unsigned integers to make it useful. This may require some custom implementation such as hifitime::StdDuration::from_hours() or hifitime::StdDuration::from_seconds(10). I am pretty sure I cannot just use hifitime::Duration since it's re-exported.

[ChristopherRabotin]

There's currently a basic from_hours implementation which will be made obsolete if and once rust-lang/rust#47097 is merged.