grow(minSize = 0) {
    let newSize = this.buf.byteLength * 4
    while (newSize < minSize) newSize *= 2
    const newBuf = new Uint8Array(newSize)
    newBuf.set(this.buf, 0)
    this.buf = newBuf
    return this
  }
Reallocates the encoder's buffer to be bigger.