Add versioned, ABI-tagged inline namespace and namespace macros (#3590)

* Add versioned inline namespace

Add a versioned inline namespace to prevent ABI issues when linking code
using multiple library versions.

* Add namespace macros

* Encode ABI information in inline namespace

Add _diag suffix to inline namespace if JSON_DIAGNOSTICS is enabled, and
_ldvcmp suffix if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is enabled.

* Move ABI-affecting macros into abi_macros.hpp

* Move std_fs namespace definition into std_fs.hpp

* Remove std_fs namespace from unit test

* Format more files in tests directory

* Add unit tests

* Update documentation

* Fix GDB pretty printer

* fixup! Add namespace macros

* Derive ABI prefix from NLOHMANN_JSON_VERSION_*

include/nlohmann/detail/iterators/iterator_traits.hpp

@@ -10,13 +10,14 @@
 
 #include <iterator> // random_access_iterator_tag
 
+#include <nlohmann/detail/abi_macros.hpp>
 #include <nlohmann/detail/meta/void_t.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 template<typename It, typename = void>
 struct iterator_types {};
 
@@ -55,5 +56,6 @@ struct iterator_traits<T*, enable_if_t<std::is_object<T>::value>>
     using pointer = T*;
     using reference = T&;
 };
-} // namespace detail
-} // namespace nlohmann
+
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END