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/json_fwd.hpp

@@ -15,13 +15,15 @@
 #include <string> // string
 #include <vector> // vector
 
+#include <nlohmann/detail/abi_macros.hpp>
+
 /*!
 @brief namespace for Niels Lohmann
 @see https://github.com/nlohmann
 @since version 1.0.0
 */
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
+
 /*!
 @brief default JSONSerializer template argument
 
@@ -67,6 +69,6 @@ struct ordered_map;
 /// @sa https://json.nlohmann.me/api/ordered_json/
 using ordered_json = basic_json<nlohmann::ordered_map>;
 
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END
 
 #endif  // INCLUDE_NLOHMANN_JSON_FWD_HPP_