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.
Put `Duration` in `time::duration`, where the two constants can
be called just `MAX` and `MIN`. Reexport from `time`.
This provides more room for the time module to expand.
