Re: 'infinity'::Interval should be added

> so I was thinking that
> postgres=# select 'infinity'::timestamp - 'infinity'::timestamp;
> would be zero rather than an error, for least surprise.

Wrong.  This case needs to be undefined, because "infinity"
isn't a specific value.  That's what makes it okay to define, say,
infinity plus any finite value as infinity.  There are very
well-defined rules about how to calculate with infinity, and
not following them is not the way to proceed here.

tl;dr: we should model it after the behavior of IEEE float infinities,
except we'll want to throw errors where those produce NaNs.