Yet another attempt to refactor libstd's Duration

[1-more]

At the moment the Duration type looks like

pub struct Duraion {
    secs: i64,
    nanos: i32
}

According to this, one can expect that maximum possible duration value is at least i64::MAX seconds. But in fact, the full range that i64 seconds gives us is restricted to i64::MAX milliseconds (see #16626 for more details).

I offer to change the internal representation of Duration to

pub struct Duraion {
    ticks: i64,  // A single tick represents 100 nanoseconds  
    nanos: i8  // to hold values from -99 to 99
}

Some pros:

1. Internals of struct Duraion are not exposed. So, we are free to change them.
2. New representation is 3 bytes smaller.
3. Methods like num_milliseconds etc will return correct i64 values (see libstd: Refactor Duration. #16626).
4. TimeSpan in .NET and Duration in Joda-Time types do the same.

Some cons:

1. The overall range of values for Duration will be shorter:
   • i64::MAX seconds gives us 106751991167300 days. We don't use need it.
   • i64::MAX milliseconds gives us 106751991167 days. This is our current choice.
   • i64::MAX ticks gives us 10675199 days. The range is still long enough, who cares?
2. ?

[jooert]

Isn't this obsolete because #24920 has been merged?

[alexcrichton]

Ah yes, thanks @jooert! More design info can also be found in the associated RFC thread.