std::time::duration::Duration::span() is backwards

[ben0x539]

Duration::span() returns negative durations!

cc #18858

[alexcrichton]

Oh dear, I'm so sorry! I'll fix pronto

[ben0x539]

Oh, don't worry about "pronto". By current reckoning you'll get the fix in with less than zero delay, I couldn't ask for more than that!