[5.0] Add where method to collection

This PR replaces #6502 and now only support simple match filtering. No more operator checking.

Usage is still roughly the same:

```php
$collection = new Collection(['v' => 1], ['v' => 2], ['v' => 3]);

$two = $collection->where('v', 2);
```

---

This is quite useful as is. If we add Eloquent support to `data_get` (which we should anyhow do, for many other reasons) this would become way more useful:

```php
if ($orders->contains('user_id', Auth::id())
{
    // 
}
```