libtime: Use Duration in Timespec arithmetic.

[ruuda]

This changes the Add and Sub implementations for Timespec introduced in #16573 to use Duration as the time span type instead of Timespec itself, as suggested by @sfackler.

This depends on #16626, because is uses Duration::seconds(i64), whereas currently Duration::seconds takes an i32.

[alexcrichton]

This looks great, thanks! It may take #16626 more time to land than this, so perhaps this could be rebased on master and this could land first?

[ruuda]

This works with the current Duration. It’ll overflow for timespecs more than about 68 years apart.

[alexcrichton]

Could you align this .num_nanoseconds() to the right to not be left-aligned?

[ruuda]

I wondered about that, what the best style is (I could not find anything in the style guide). Do you mean aligning it with the opening parenthesis?

[alexcrichton]

Now that #16626 has been approve, this should probably wait to get merged, other than that this looks good to me!

[alexcrichton]

Now that #16626 has merged, should this be rebased to remove the FIXMEs?

[ruuda]

Yes, I’m running make check at the moment.