Add read() method to ReadableByteStream

Oh, I see. I didn't appreciate the difference between [[readBufferSize]] and bytesRead. Interesting.

I don't like the fits-perfectly idea. The point of having the constructor should be so that if we had read(2) exposed directly to JavaScript, we could wrap it in a ReadableByteStream. It sounds like the fits-perfectly idea would end up bypassing the [[readBufferSize]] constructor argument.

Do you think [[readBufferSize]] might not be the best approach? Or maybe it is good for a default, but we should allow a custom read() alternative for the underlying source?

All that said, I am totally fine with Uint8Array (or maybe [DataView](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView)?) now that I realize the difference. I just want to make sure we are thinking this through.