This may also work with later versions, but I did not test them

It's a bit clumsy to construct a Bitvec instance when the bit width is
not statically known, e.g.:

```
let add n x y =
  let module M = Bitvec.Make(struct
    let modulus = Bitvec.modulus n
  end) in
  M.(x + y)
```

Instead, we can create a first-class module more concisely:

```
let add n x y =
  let open (val Bitvec.modular n) in
  x + y
```

Unrelatedly, this also adds the missing `Bitvec.M16` specialization and
its corresponding modulus.