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
Unlike my previous comment, the name of the variable actually is part of the public documentation for the method, so I would prefer to see this take "nsecs". Alternatively, as this is public documentation, perhaps the abbreviations should be dropped, as in

```rust
pub fn new(days: i32, seconds: i32, nanoseconds: i32) -> Option<Duration>
```