writeOnDataView = (encoder, len) => {
  verifyLen(encoder, len);
  const dview = new DataView(encoder.cbuf.buffer, encoder.cpos, len);
  encoder.cpos += len;
  return dview;
}
Create an DataView of the next `len` bytes. Use it to write data after
calling this function.

```js
// write float32 using DataView
const dv = writeOnDataView(encoder, 4)
dv.setFloat32(0, 1.1)
// read float32 using DataView
const dv = readFromDataView(encoder, 4)
dv.getFloat32(0) // => 1.100000023841858 (leaving it to the reader to find out why this is the correct result)
```

@param {Encoder} encoder
@param {number} len
@return {DataView}