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/macro_scope.hpp

@@ -9,12 +9,14 @@
 #pragma once
 
 #include <utility> // declval, pair
-#include <nlohmann/thirdparty/hedley/hedley.hpp>
 #include <nlohmann/detail/meta/detected.hpp>
+#include <nlohmann/thirdparty/hedley/hedley.hpp>
 
-// This file contains all internal macro definitions
+// This file contains all internal macro definitions (except those affecting ABI)
 // You MUST include macro_unscope.hpp at the end of json.hpp to undef all of them
 
+#include <nlohmann/detail/abi_macros.hpp>
+
 // exclude unsupported compilers
 #if !defined(JSON_SKIP_UNSUPPORTED_COMPILER_CHECK)
     #if defined(__clang__)
@@ -457,14 +459,6 @@
     #define JSON_EXPLICIT explicit
 #endif
 
-#ifndef JSON_DIAGNOSTICS
-    #define JSON_DIAGNOSTICS 0
-#endif
-
-#ifndef JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
-    #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0
-#endif
-
 #ifndef JSON_DISABLE_ENUM_SERIALIZATION
     #define JSON_DISABLE_ENUM_SERIALIZATION 0
 #endif