Add support for reading and writing elements as raw, pre-encoded bytes

Apparently there are use cases like the one described in #761 that need this.
The proposal is to add the following method to `InputReader`:

```scala
def rawBytesOfLastValue(preElementCursor: Int): Array[Byte]
```

and this method to `Writer`"

```scala
def writeValueAsRawBytes(bytes: Array[Byte]): Self
```
