appendRawBytes(data) {
    if (this.offset + data.byteLength > this.buf.byteLength) {
      this.grow(this.offset + data.byteLength)
    }
    this.buf.set(data, this.offset)
    this.offset += data.byteLength
    return data.byteLength
  }
Appends the contents of byte buffer `data` to the buffer. Returns the
number of bytes appended.