Add ExpectArrayLen / ReadFixedArray

[vmihailenco]

func (r *Reader) ReadFixedArray(caller string, n int) error {
	got, err := r.ReadArrayLen()
	if err != nil {
		return err
	}
	if n != got {
		return fmt.Errorf("redis: %s: unexpected array length; want %d, got %d", caller, n, got)
	}
	return nil
}

[icholy]

I was thinking about the lack of context information in the error too. My reasoning for leaving it out is that it's not something that should ever happen. When you hit this error, it indicates a bug in the library (not user code). An incorrect number of array elements is no different than trying to read an int reply instead of a string reply. Yet those methods also don't have a "caller" parameter. If we want to add context to the error, we should do it consistently further up the call chain.

[vmihailenco]

It is a bug in the library and users will report it here. Without context I have no idea where it happens...

[vmihailenco]

Fixed by #1739