chore: move streams/buffer.ts to io/buffer_stream.ts

[crowlKats]

Since we are doing #1986, it would be weird to have all the stream versions of things in the streams folder instead of the corresponding folder

[nayeemrmn]

This would break import { Buffer } from "...std/io/mod.ts"; assuming io/buffer_stream.ts was added there. You can rename it to StreamBuffer or something, but I think the better solution is to keep it in streams with the intention of streams slowly replacing io. We've mostly already gone in that direction.

[crowlKats]

yea, thats true. will close this then.