Implement new array function array_column()

array_column() returns the values of the specified column from a multi-dimensional array.

I've actually had this patch sitting around for several years, and I decided to take a look, clean it up, and finally submit it. Inspired by database methods like PDOStatement::fetchColumn(), I wrote this to scratch an itch, since I've often had the need to take a recordset array and pull a single column of values from it, which normally involves looping over the array and creating a new array of the values for the column I want to pull. I decided to implement this in the PHP core to save that step.

You can take a look at the test included to see how it works. The second parameter is the key for the column to return, which can be a number for numeric indexes or a string for associative arrays. If it can't find the column specified, it simply returns an empty array.