Write a function named array_split that accepts an array of integers as a
parameter and returns a new array twice as large as the original, replacing every
integer from the original array with a pair of integers, each half the original.
For example, if a variable named $arr refers to an array storing the values
[18, 7, 4, 24, 11], the call of array_split($arr) should return
a new array containing [9, 9, 3.5, 3.5, 2, 2, 12, 12, 5.5, 5.5].
(The number 18 is split into the pair 9, 9, the number 7 is split into 3.5, 3.5,
the number 4 is split into 2, 2, the number 24 is split into 12, 12 and the number
11 is split into 5.5, 5.5.)
