Reversing a string by copy and in place

Write a function that returns a reversed copy of a string, and a second one that reverses the string where it is, without allocating memory.

Copy: build from reverse iterators, $O(n)$ time and $O(n)$ space. In place: swap characters from both ends moving inward until the indices meet, $O(n)$ time, $O(1)$ extra space. Take the in-place version by non-const reference, handle empty and odd-length strings, and note that byte-level reversal breaks multi-byte encodings such as UTF-8.