Refactor the simplifier a bit to fix #22761

The core change in this commit, which fixes #22761 (closed), 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.