libstd: Refactor Duration.

This changes the internal representation of `Duration` from

    days: i32,
    secs: i32,
    nanos: u32

to

    secs: i64,
    nanos: i32

This resolves #16466. Some methods now take `i64` instead of `i32` due
to the increased range. Some methods, like `num_milliseconds`, now
return an `Option<i64>` instead of `i64`, because the range of
`Duration` is now larger than e.g. 2^63 milliseconds.