Length in unboxed instances

`Unbox` instances for tuples (defined in `unbox-tuple-instances`) are backed by vectors of the form:
```haskell
data instance Vector (a, b)
= V_2 {-# UNPACK #-} !Int !(Vector a) !(Vector b)
```

This stores the length of each vector. Is there a fundamental reason why we need this length? It seems, naively, like it wastes an extra word as each vector already contains the length and an invariant of the representation should be that both lengths remain identical.