Write a function stutter that takes an array of elements as a parameter
and that replaces every element with two of occurrences of that element.
For example, if an array stores the values ["how", "are", "you?"] before the
function is called, it should store the values ["how", "how", "are", "are", "you?", "you?"]
after the function finishes executing.
If the array instead stored [0, 0, 1], it should store the values
[0, 0, 0, 0, 1, 1] after the function finishes executing.
A note about references in PHP: In order to write a function passes a parameter as
reference (thus modifying its state), you'll need to prepend "&" to the variable declaration
in the function header. For example, a function foo that modifies the state
of an array parameter may be defined as:
function foo(&$arr) { ... }
