Re: Fix overflow in DecodeInterval

Attached is a patch of my first pass. The to_char method isn't finished
and I need to add a bunch of tests, but everything else is in place. It
ended up being a fairly large change in case anyone wants to take a look
the changes so far.

One thing I noticed is that interval.c has a ton of code copied and pasted
from other files. The code seemed out of date from those other files, so
I tried to bring it up to date and add my changes.

- Joe Koshakow