Add InstantEx trait to support std::time::Instant operations.

[osamasalem]

Hey there 👋

I just added a couple of functions to the repo to support std::time::Instant operations, it is not my idea, we credit DHH for !! 😄

it is something like

let five_secs_ago = 5.seconds().ago();

So it returns an instant from 5 secs before now()

Let me know if anything else needed from this PR

Thansks

[JensMertelmeyer]

I think you could add some documentation about when to expect a runtime panic.

[durationextender]

Nice suggestion i will take a look at it!