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.