Add dump method to Collection class

[freekmurze]

This PR adds a dump method to Illuminate\Support\Collection so you can easily debug multiple spots in a collection chain.

The function accepts and dumps a variable number of arguments. This can be used to add labels in the output when dumping at multiple spots in a collection chain. Example of that behaviour can be found here: https://murze.be/2016/08/debugging-collection-chains/