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
This is a bit misleading. It sounds like `Duration::Days(1).num_hours()` should return `24` but it returns `0`.

Especially since the wording here matches the wording on `num_weeks()`/`num_days()`, yet `Duration::weeks(1).num_days()` returns `7` instead of `0`.