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/conversions/from_json.hpp

@@ -24,28 +24,15 @@
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/meta/identity_tag.hpp>
+#include <nlohmann/detail/meta/std_fs.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
-#if JSON_HAS_EXPERIMENTAL_FILESYSTEM
-#include <experimental/filesystem>
-namespace nlohmann::detail
-{
-namespace std_fs = std::experimental::filesystem;
-} // namespace nlohmann::detail
-#elif JSON_HAS_FILESYSTEM
-#include <filesystem>
-namespace nlohmann::detail
-{
-namespace std_fs = std::filesystem;
-} // namespace nlohmann::detail
-#endif
-
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 template<typename BasicJsonType>
 inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
 {
@@ -491,6 +478,7 @@ struct from_json_fn
         return from_json(j, std::forward<T>(val));
     }
 };
+
 }  // namespace detail
 
 #ifndef JSON_HAS_CPP_17
@@ -503,6 +491,7 @@ namespace // NOLINT(cert-dcl59-cpp,fuchsia-header-anon-namespaces,google-build-n
 JSON_INLINE_VARIABLE constexpr const auto& from_json = // NOLINT(misc-definitions-in-headers)
     detail::static_const<detail::from_json_fn>::value;
 #ifndef JSON_HAS_CPP_17
-} // namespace
+}  // namespace
 #endif
-} // namespace nlohmann
+
+NLOHMANN_JSON_NAMESPACE_END