This patch nails a Bad Bug exposed in Trac #13379. Roughly,
a deeply-nested application like
   f (f (f ....) ) )
could make the simplifier go exponential -- without producing
an exponential-sized result!

The reason was that we
  - simplified a (big) function argument
  - then decided to inline the function
  - then preInilneUnconditionally the argument
  - and then re-simplified the big argument

And if the "big argument" itself had a similar structure
things could get very bad.

Once I'd understood, it was easy to fix:

* See Note Note [Avoiding exponential behaviour] for an overview

* The key change is that Simplify.simplLam now as a case for
  (isSimplified dup). This is what removes the perf bug.

* But I also made simplCast more parsimonious about simplifying,
  avoiding doing so when the coercion is Refl

* And similarly I now try to avoid simplifying arguments
  where possible before applying rules.
  See Note [Trying rewrite rules]

The latter two points tackle common cases, and in those cases make the
simplifier take fewer iterations.