DmdAnal: Unleash demand signatures of free RULE and unfolding binders (#23208)

In #23208 we observed that the demand signature of a binder occuring in a RULE
wasn't unleashed, leading to a transitively used binder being discarded as
absent. The solution was to use the same code path that we already use for
handling exported bindings.

See the changes to `Note [Absence analysis for stable unfoldings and RULES]`
for more details.

I took the chance to factor out the old notion of a `PlusDmdArg` (a pair of a
`VarEnv Demand` and a `Divergence`) into `DmdEnv`, which fits nicely into our
existing framework. As a result, I had to touch quite a few places in the code.

This refactoring exposed a few small bugs around correct handling of bottoming
demand environments. As a result, some strictness signatures now mention uniques
that weren't there before which caused test output changes to T13143, T19969 and
T22112. But these tests compared whole -ddump-simpl listings which is a very
fragile thing to begin with. I changed what exactly they test for based on the
symptoms in the corresponding issues.

There is a single regression in T18894 because we are more conservative around
stable unfoldings now. Unfortunately it is not easily fixed; let's wait until
there is a concrete motivation before invest more time.

Fixes #23208.