Disallow fixed-vector-2

It breaks the build of hgeometry with this error:
> src/Data/Geometry/Vector/VectorFixed.hs:110:34: error: [GHC-83865]
>   • Couldn't match kind ‘Natural’ with ‘V.PeanoNum’
>     Expected kind ‘V.PeanoNum’, but ‘d’ has kind ‘Nat’
>   • In the type ‘d’
>     In the type family instance declaration for ‘V.Dim’
>   |
> 110 | type instance V.Dim (Vector d) = d