Improve performance (throughput)

[lambdalisue]

Personally, I feel that the complexity of a few hundred milliseconds for 200,000 entries is not worth the cost. Therefore, I will leave it to you to decide whether to merge them.

When using async iterator, there seems to be a problem that iterating on individual elements affects performance.
In other words, it's likely that Deno has the same problem as the following issue that exists in Node.

nodejs/node#31979

So I tried to avoid this problem by using AsyncGenerator<T[]> instead and seems working.

Results of {'profile': 1}

I've made 200,000 files with

#!/bin/bash
mkdir -p out
for i in {1..200000}; do
  touch out/$i.txt
done

Then run call ddu#start({'profile': 1}) on out directory.

Before #3	After #3	This PR
1167 ms	1591 ms	1213 ms
1217 ms	1569 ms	1165 ms
1165 ms	1588 ms	1148 ms
1186 ms	1562 ms	1155 ms
1121 ms	1493 ms	1151 ms

[Shougo]

Hi, I have tested the branch. It seems 5%~10% faster than main. But it is not so huge difference...

[Shougo]

I will merge it. I think it is better than previous version and abortable merge is needed.
Thanks.