Significant performance impact

It's too slow, unfortunately.

The Array.forEach call is too expensive.

Also, organize the numeric settings a bit on the ReadableState class

It was testing the length *before* adding the current chunk, which
is the opposite of correct.

Also, the return value was flipped.

That is, the transform is triggered by a _read, not by a _write.

This way, backpressure works properly.