Migrate to illuminate/collections

Your README states that you depend on `Tightenco\Collect`, but that package has been deprecated in favour of the official upstream `illuminate/collections` package (which is also provided `by laravel/framework`). Curiously, you don't even seem to use that package in favour of your own `pragmarx/ia-collection`, which could also be deprecated as far as I can tell.

This would be nice to reduce the amount of duplicate code I'm pulling in a Laravel project, and certainly be easier for you to maintain without having to check for and pull in any upstream changes all the time. Mostly though it's just a pet-peeve that I've always got to search for the correct Collection/Arr/Str classes to use when I just want the official ones.

Start with the README references to Tightenco\Collect and inspect where pragmarx/ia-collection is used. Compare those references with illuminate/collections before defining the migration scope; done means the deprecated and duplicate collection dependencies are replaced consistently and the README identifies the supported upstream package.

Written by the indexing model from the issue text.