Exercise: palindrome

Write a function bool is_palindrome(std::string_view s). It returns true if s is a palindrome, and false otherwise. A palindrome reads the same from left to right and from right to left.

• is_palindrome("abba") → true
• is_palindrome("level") → true
• is_palindrome("abca") → false

Constraints: letter case matters, so "Abba" is not a palindrome; an empty string and a one-character string are palindromes.

Do not add extra #include lines unless the task says so explicitly.