Re: [streams] Add .bytesRead to ReadableByteStream.*Reader (#367)

I'm fine with `offset`.

- The good point of `offset` and `fooCount` is that it sounds like holding an accumulated historical data.
- The bad point of `count` is that it sounds like counting objects, not the result of `size()`
- `offset` may sound like it's settable to perform seeking, but it doesn't matter much.

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