Rename all Duration to TimeDelta

[fsktom]

In chrono (currently, 0.4.35) Duration is just a type alias for TimeDelta (see chronotope/chrono#1137 (comment))

I think it'd be better if I use the actual struct name instead of its alias (also to differentiate it from std's Duration)

[fsktom]

https://docs.rs/chrono/0.4.38/chrono/index.html#time-delta--duration

The TimeDelta type was previously named Duration (and is still available as a type alias with that name). A notable difference with the similar core::time::Duration is that it is a signed value instead of unsigned.