Avoid including <iostream> in library code

Including iostream means introducing the static (global) constructors
and destructors for std::cin, std::cerr, and std::cout. That extra
init and fini code is undesirable when those streams are not actually
used.

Instead, we'll use the narrower includes for exactly what's needed,
i.e., <istream> or <ostream>.

src/indentation.h

@@ -7,7 +7,6 @@
 #pragma once
 #endif
 
-#include <iostream>
 #include <cstddef>
 
 #include "yaml-cpp/ostream_wrapper.h"