join() {
        // get chunk of fork
        let chunk = this.finish();
        // restore previous state
        let prev = this.stack.pop();
        if (!prev)
            throw new Error('invalid state, fork stack empty');
        this.chunks = prev.chunks;
        this.buf = prev.buf;
        // write length of chunk as varint
        this.uint32(chunk.byteLength);
        return this.raw(chunk);
    }
Join the last fork. Write its length and bytes, then
return to the previous state.