Fix stale #include lines in the output headers

output_adapters.hpp included <algorithm> and <iterator> for std::copy
and std::back_inserter, which have not been used there since #3569
(2022). serializer.hpp included <algorithm> for std::reverse (also
unused), <cmath> for labs/isnan/signbit (only std::isfinite is used)
and <utility> for std::move (nothing from <utility> is used there),
while using std::next without including <iterator> at all, relying on
getting it transitively through output_adapters.hpp's own stale
<iterator>.

Drop the unused includes, add <iterator> for std::next, and correct
the remaining include comments. Both headers still compile standalone
with -Wall -Wextra -Werror.

Part of #5709

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/output/output_adapters.hpp

@@ -8,9 +8,7 @@
 
 #pragma once
 
-#include <algorithm> // copy
 #include <cstddef> // size_t
-#include <iterator> // back_inserter
 #include <memory> // shared_ptr, make_shared
 #include <string> // basic_string
 #include <utility> // move

include/nlohmann/detail/output/serializer.hpp

@@ -9,18 +9,18 @@
 
 #pragma once
 
-#include <algorithm> // reverse, remove, fill, find, none_of, min
+#include <algorithm> // remove, fill, find, none_of, min
 #include <array> // array
 #include <clocale> // localeconv, lconv
-#include <cmath> // labs, isfinite, isnan, signbit
+#include <cmath> // isfinite
 #include <cstddef> // size_t, ptrdiff_t
 #include <cstdint> // uint8_t
 #include <cstdio> // snprintf
 #include <cstring> // memcpy, memset
+#include <iterator> // next
 #include <limits> // numeric_limits
 #include <string> // string, char_traits
 #include <type_traits> // is_same
-#include <utility> // move
 #include <vector> // vector
 
 #include <nlohmann/detail/conversions/to_chars.hpp>

single_include/nlohmann/json.hpp

@@ -20146,9 +20146,7 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
-#include <algorithm> // copy
 #include <cstddef> // size_t
-#include <iterator> // back_inserter
 #include <memory> // shared_ptr, make_shared
 #include <string> // basic_string
 #include <utility> // move
@@ -22960,18 +22958,18 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
-#include <algorithm> // reverse, remove, fill, find, none_of, min
+#include <algorithm> // remove, fill, find, none_of, min
 #include <array> // array
 #include <clocale> // localeconv, lconv
-#include <cmath> // labs, isfinite, isnan, signbit
+#include <cmath> // isfinite
 #include <cstddef> // size_t, ptrdiff_t
 #include <cstdint> // uint8_t
 #include <cstdio> // snprintf
 #include <cstring> // memcpy, memset
+#include <iterator> // next
 #include <limits> // numeric_limits
 #include <string> // string, char_traits
 #include <type_traits> // is_same
-#include <utility> // move
 #include <vector> // vector
 
 // #include <nlohmann/detail/conversions/to_chars.hpp>