CorePrep: Refactor FloatingBind (#23442)

A drastically improved architecture for local floating in CorePrep
that decouples the decision of whether a float is going to be let- or case-bound
from how far it can float (out of strict contexts, out of lazy contexts, to
top-level).

There are a couple of new Notes describing the effort:

  * `Note [Floating in CorePrep]` for the overview
  * `Note [BindInfo and FloatInfo]` for the new classification of floats
  * `Note [Floats and FloatDecision]` for how FloatInfo is used to inform
    floating decisions

This is necessary ground work for proper treatment of Strict fields and
unlifted values at top-level.

Fixes #23442.