Refactor the simplifier a bit to fix #22761

The core change in this commit, which fixes #22761, is that

* In a Core rule, ru_rhs is always occ-analysed.

This means adding a couple of calls to occurAnalyseExpr when
building a Rule, in
* GHC.Core.Rules.mkRule
* GHC.Core.Opt.Simplify.Iteration.simplRules

But diagosing the bug made me stare carefully at the code of the
Simplifier, and I ended up doing some only-loosely-related refactoring.

* I think that RULES could be lost because not every code path
  did addBndrRules

* The code around lambdas was very convoluted

It's mainly moving deck chairs around, but I like it more now.