Minor optimizations to array_keys()/array_values()

Minor optimizations to array_keys()/array_values()

array_values():
When the input is an empty array or a packed array with no gaps,
return the original array.

array_keys():
When the input is an empty array, return the original array.
When the input is a packed array with no holes
(and no search key specified), populate the return with
a simple range(0, count($input) - 1)