mixed *map_array(mixed *arr, string fun, object ob, mixed extra)

Returns an array holding the items of 'arr' mapped through ob->fun().
The function 'fun' in 'ob' is called for each element in 'arr' with that
element as parameter. A second parameter 'extra' is sent in each call if
given. Principal function: 
	foreach (index) arr[index] = ob->fun(arr[index],extra);

The value returned by ob->fun(arr[.index.], extra) replaces the existing 
element in the array.
