[9.x] Adds Arr::map and Arr::transform

[ecrmnn]

Adds Arr::map and Arr::transform.

I've finally created a PR for this after assuming it's there for a while.
Everytime I realize it's not, I'm wrapping my array in a collection to get that sweet syntactic sugar 🍬

• Significantly better syntax than array_map
• Iterates both values and keys out of the box
• Consistent API with Collection
• Refactored Collection to consume Arr::map

Added Arr::transform because they're basically the same, except $array is passed by reference.