add platform interface & refactor all Instant to Duration

[zhangzqs]

No description provided.

[maxwase]

Thank you for the PR! I've looked into it, but didn't quite understand the idea behind it, here some of my thoughts:

1. Usually there is no need for an abstraction to know which platform it runs on. Here I see how you propose to move the Instant type out of State, however later the crate still needs an Instant or it's analogue to compute the duration, which to me sounds like an extra step to get the elapsed value.
2. I understand that you're proposing an abstraction that aims to allow user to choose how to implement elapsed, however IHMO the crate needs an Instant::now abstraction because elapsed is kind of fixed, it's always now - self, we just need a way to know the current time.

I'll open a PR shortly to implement the way I see it, a review from you would be appreciated as I don't really know your use case