[reactive] I think I found the bug with joinE/mappend

[Conal Elliott]

I think I finally found the cause of a key laziness bug in reactive.  It
took me a long time to track down.  The way I structured the AddBounds type
stops me from getting partial info I need.  There's a critical max on times
in joinE.  It'll be 'max (NoBound t0) ta', and I know that ta is either a
NoBound or a MaxBound, but without knowing which, no info can come out,
which stops the consumer of that max from knowing that the time is at least
NoBound t0.  Now I think I have to make some pervasive changes to reactive.

I'd like to develop the QC tests that will catch these mistakes much
earlier.   I recently read the "Chasing Bottoms" paper, which some good
techniques.

   - Conal
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