Add padding via codec to fixed size codecs

Partially implements #21 

Only pads to the right

Has "wild card" scalaz import as to use ordering.
Perhaps you know what subset suffices?
Tail recursive version:

```scala
@tailrec def pad(bits: BitVector): String \/ BitVector = bits.size ?|? size match {
  case Ordering.EQ => \/.right(bits)
  case Ordering.LT => padCodec.encode(()) match {
    case \/-(p) => pad(bits ++ p)
    case e @ -\/(_) => e
  }
  case Ordering.GT => \/.left(s"[$a] padded by [$padCodec] overflows fixed size field of $size bits")
}
```