This patch fixes Trac #10148, an outright and egregious
bug in the demand analyser.

It is explained in Note [Demand on case-alternative binders]
in Demand.hs.

I did some other minor refactoring.

To my astonishment I got some big compiler perf changes

* perf/compiler/T5837: bytes allocated -76%
* perf/compiler/T5030: bytes allocated -10%
* perf/compiler/T3294: max bytes used  -25%

Happy days