Method names and functionalities are inspired by [Kotlin.Collections](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/), having a slightly different style than that of [Laravel Collections](https://laravel.com/docs/5.8/collections).
* The `Collection::xxxTo()` methods will preserve the original key-value pairs of destination `Collection` when keys collide.
* Some methods of `Collection` involves comparing two of its elements, which accepts `$flags` as one of its arguments. When these methods are being invoked, make sure all elements are of the same type (numeric/string/others), otherwise you're likely to get a segfault.
Class `Collection` does not introduce new data structures internally. Instead, it only holds a pointer to a `zend_array`, and all its methods works directly on top of `zend_array`. Which means conversion between `Collection` and `array` does not involve copying, until write operation is performed on one of the duplicates.
```php
$foo = ['a', 'b']; // arr0: refcount = 1
$bar = Collection::init($foo); // arr0: refcount = 2, no copying of either `zend_array` or its elements