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/adl_serializer.hpp

@@ -8,16 +8,14 @@
 
 #pragma once
 
-#include <type_traits>
 #include <utility>
 
+#include <nlohmann/detail/abi_macros.hpp>
 #include <nlohmann/detail/conversions/from_json.hpp>
 #include <nlohmann/detail/conversions/to_json.hpp>
 #include <nlohmann/detail/meta/identity_tag.hpp>
-#include <nlohmann/detail/meta/type_traits.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 
 /// @sa https://json.nlohmann.me/api/adl_serializer/
 template<typename ValueType, typename>
@@ -53,4 +51,5 @@ struct adl_serializer
         ::nlohmann::to_json(j, std::forward<TargetType>(val));
     }
 };
-}  // namespace nlohmann
+
+NLOHMANN_JSON_NAMESPACE_END