Duration should never have checked for division by 0 and thrown a
TimeException when it occurred. The OS/hardware already checks for that,
and it should be considered a logic error. Checking for it and possibly
throwing just slows the code down and makes it so that it can't be @nogc.

This is technically a breaking change, but it will only break code which
doesn't bother to avoid dividing by 0 and then catches TimeException
when it occurs, and such code is likely extremely rare, if it exists at
all. Given the fact that the odds of actually breaking code are
extremely low and that this allows us to make Duration fully @nogc except
for toString, I think that it's worth the risk.

Treat division by 0 as an error in Duration and consolidate code.