Convert function_exists() to is_callable()

@quietone Should we replace all existences of function_exists() with is_callable() ?

@annmarysruthy, I think each usage needs to be examined. For example, the usages in tests should probably not change, In may turn out that there are no places where making this change is an improvement. I have not looked closely at all the instances.

Checked all occurrences of function_exists() and changed 6 files. In my opinion, Rest of the occurrences are better kept as function_exists() itself. Kindly review MR

This looks great! I have to review more closely, but this already covers the three I was reviewing.

curious to compare performance of both methods