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/json_ref.hpp

@@ -11,12 +11,13 @@
 #include <initializer_list>
 #include <utility>
 
+#include <nlohmann/detail/abi_macros.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 template<typename BasicJsonType>
 class json_ref
 {
@@ -72,5 +73,6 @@ class json_ref
     mutable value_type owned_value = nullptr;
     value_type const* value_ref = nullptr;
 };
+
 }  // namespace detail
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END