Unexpected results given very large sizes

Since file sizes are stored in bytes, the utility is limited to dealing in byte counts up to `PHP_INT_MAX`. So when the maximum integer is `9223372036854775807`, this means the largest possible size is 8 exabytes. An example test:

```php
$size = new FileSize('8.1 EB');
$this->assertSame($size->asAuto(), '8.1 EB');
```

```bash
--- Expected
+++ Actual
@@ @@
-'-7.9 EB'
+'8.1 EB'
```