Implement swap function (Bug #70736)

[jerrygrey]

See Bug #70736

[michaeljs1990]

This seems like a unnecessary function. Outside of the simpleness of just using...

list($one, $two) = array($two, $one);

I looked around and can't find any other language with a built in function for this. Not that this is reason alone for exclusion it is a good heuristic that likely means this is not an issue and doesn't need to be solved.

[jerrygrey]

@michaeljs1990 I wouldn't say using list() is simple, it is not really clear what is happening at a glance, but I agree that there isn't really a clear need for the function.

Plus, I have no clue why Travis is failing at the moment.