Some polishing of the demand analyser.
I did some refactoring of the demand analyser, because I was smelling some minor code smell. Most of my changes I had to undo, though, adding notes and testcases on why the existing code was correct after all. Especially the semantics of the DmdResult is confusing, as it differs in a DmdType and a StrictSig. I got to imrpove the readability of the code for lubDmdType, though. Also, dmdAnalRhs was a bit fishy in how it removed the demand on further arguments of the body, but used the DmdResult. This would be wrong if a body would return a demand type of "<L>m" (which currently does not happen). This is now treated better in removeDmdTyArgs.
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