Remove dependency on lens-family
Use `decodedL` lens in `decoded` to avoid code duplication
Make `decodedL` a proper `Iso'`
Revert back to independant `decodeL` and `decode`

I was looking for an isomorphism between a producer streaming `a`
and (ByteOffset, a), but couldn't come with a straightforward one.

Now it's possible to drop `lens-family` dependency.