This patch collects a few improvements triggered by Trac #15696,
and fixing Trac #16029
* Stop making toCleanDmd behave specially for unlifted types.
This special case was the cause of stupid behaviour in Trac
#16029. And to my joy I discovered the let/app invariant
rendered it unnecessary. (Maybe the special case pre-dated
the let/app invariant.)
Result: less special-case handling in the compiler, and
better perf for the compiled code.
* In WwLib.mkWWstr_one, treat seqDmd like U(AAA). It was not
being so treated before, which again led to stupid code.
* Update and improve Notes
There are .stderr test wibbles because we get slightly different
strictness signatures for an argumment of unlifted type:
<L,U> rather than <S,U> for Int#
<S,U> rather than <S(S),U(U)> for Int