This implements the BoxedRep proposal, refactoring the `RuntimeRep`
hierarchy from:

```haskell
data RuntimeRep = LiftedPtrRep | UnliftedPtrRep | ...
```

to

```haskell
data RuntimeRep = BoxedRep Levity | ...
data Levity = Lifted | Unlifted
```

Updates binary, haddock submodules.

Closes #17526.

Metric Increase:
    T12545