Implement Nested CPR

This ticket tracks the design and implementation of a Nested CPR, e.g. extending CPR analysis to unbox constructed products nested inside constructed products.

A list of tickets for motivation:

CPR IO/State computations #1600 (closed)
Various places in GHC/base where we currently unbox IO manually, like GHC.Utils.Encoding or UniqSM
A simple rapid-termination analysis (higher-order exprIsCheap/exprOkForSpeculation) #8655
Effective sum CPR needs to be nested and hence the terminaion analysis machinery
Many heuristics of our current CPR analysis (i.e. Note [CPR for binders that will be unboxed]) are an artifact of not being able to express and propagate nested CPRs in case scrutinees to field binders.

Edited Sep 30, 2021 by Sebastian Graf

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