Add missing methods to Instant, matching Rust 1.39

[fusetim]

Added some missing methods, introduced by Rust 1.39:

• Instant::checked_duration_since

  Returns the amount of time elapsed from another instant to this one, or None if that instant is later than this one.

• Instant::saturating_duration_since

  Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

• Instant::checked_add

  Returns Some(t) where t is the time self + duration if t can be represented as Instant (which means it’s inside the bounds of the underlying data structure), None otherwise.

  Since it is a floating number inside, this is always true.

• Instant::check_sub

  Returns Some(t) where t is the time self - duration if t can be represented as Instant (which means it’s inside the bounds of the underlying data structure), None otherwise.

  Since it is a floating number inside this is always true.

[mxinden]

This would allow creating a negative Instance, right?

Instant::now() - (Instant::now() + Duration::from_secs(1))

[fusetim]

It seems, I don't know for sure if it should be avoided or not.

[mxinden]

I very much think this should be avoided (across the whole crate). I do think we should mimic the behavior of the standard library here.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6e4ac52feea310c855e5a4d420e2d8ea

[fusetim]

This highly depends of the underlying implementation and could be different on an other platform. Nevertheless we could restrict the use of negative Instant.

I can't easily edit the PR for now, but I'll be back on August.