Playground to learn higher order functions in Swift :)

1. Pass function to another function
2. Return a function from another function
3. MAP - Used to apply some operation on each item in an array or dictionary
4. FILTER - Used to filter out items from an arry or dictionary based on a condition
5. REDUCE - Used to combine all items into a single new value
6. FLATMAP - Used to flatten a collection of collections
7. Chaining multiple higher order functions