Reverse word in place, character by character – the exact same converging two-pointer/two-index technique used
to reverse an int array earlier in this module, adapted to a string: find the length with strlen, swap the first
and last characters, then the second and second-to-last, and so on until the two indices meet or cross.
The null terminator itself never needs to move – it’s already sitting in the right place, one past the last real
character, before and after the reversal. Only the len characters before it get swapped.