conts, handlers, and liveness

[Matthew Fluet]

> > Now that I'm a little more familiar with the backend, I see that I'm
> > really duplicating information.  We can get by with just the jumpInfo
> > returning the various liveBegin lists and contInfo and handlerInfo just
> > having size information.  That seems a little cleaner.
>
> liveOffsets in the contInfo makes sense to me.  After all, it is only defined if
> the jump is a cont.  Maybe a better way to go would be to have a single
> property, jumpInfo, that has an option for the handler and an option for the
> cont?

That seems reasonable -- contInfo and handlerInfo are only used in a
couple of places in backend.fun.  I don't know why we have liveOffsets for
the handlers in the release version -- we never use it.