Add ExpectArrayLen / ReadFixedArray

```go
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
}
```