include/array: Improve performance of array_to_lines()

include/array: Improve performance of array_to_lines()

`array_to_lines()` calls `echo` for every element in the array, which
is wasteful and scales very poorly.

This commit modifies `array_to_lines()` so that it uses a single call
to `printf` to print all array elements.

This issue was pointed out by reddit user u/oh5nxo.