[7.x] Add support for Arr::map

[stevelacey]

Standard array_map doesn't provide access to keys within it's callback, forcing the usage of collections, loops or awkward core function combos if you want to use the key, this moves the Collection map functionality over to Arr.

[taylorotwell]

I suggest just using a collection and converting it back into an array.

[stevelacey]

Woohoo! #42398