Add a combination of countBits and nthBitIndex

Namely, 
```haskell
foo :: Bit -> Int -> Vector Bit -> Either Word Int
```
such that `foo b n vec` either returns the index of `n`-th occurence of `b` bit in `vec` or the total number of bits `b` in `vec`.

This is similar in spirit to https://hackage.haskell.org/package/text-2.1.2/docs/Data-Text.html#v:measureOff.