Note [Floats and FloatDecision]

We have a special datatype `Floats` for modelling a telescope of `FloatingBind`
and caching its "maximum" `FloatInfo`, according to `floatsAtLeastAsFarAs`
(see Note [BindInfo and FloatInfo] for the ordering).
There are several operations for creating and combining `Floats` that maintain
scoping and the cached `FloatInfo`.

When deciding whether we want to float out a `Floats` out of a binding context
such as `let x = <> in e` (let), `f <>` (app), or `x = <>; ...` (top-level),
we consult the cached `FloatInfo` of the `Floats`:

  * If we want to float to the top-level (`x = <>; ...`), we check whether
    we may float-at-least-as-far-as `TopLvlFloatable`, in which case we
    respond with `FloatAll :: FloatDecision`; otherwise we say `FloatNone`.
  * If we want to float locally (let or app), then the floating decision is
    described in Note [wantFloatLocal].

`executeFloatDecision` is then used to act on the particular `FloatDecision`.