Add Duration type to std and use it in Timer methods.

Currently, the Timer methods take an integer number of ms. This is considered a bug because a) types, b) some timers have ns precision.

This plucks the `Duration` type from [rust-chrono](https://github.com/lifthrasiir/rust-chrono), plops it into `std::time`,  and replaces the arguments to `sleep`, `oneshot`, and `periodic` timers with it. It leaves the old methods intact as `sleep_ms`, `oneshot_ms`, and `periodic_ms`, for convenience.

Closes https://github.com/rust-lang/rust/issues/11189.

cc @lifthrasiir @aturon @kballard @alexcrichton
Hm, do we actually want to be storing days here? If it's truly a duration, a day is a slightly peculiar unit since a real day is not actually a fixed length.

This could just store `secs: i64, nanos: u32` which actually covers a wider range, and doesn't suffer from the above imprecision. (I don't know if this violates ISO 8601, @lifthrasiir?)