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/input/input_adapters.hpp

@@ -26,10 +26,10 @@
 #include <nlohmann/detail/iterators/iterator_traits.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 /// the supported input formats
 enum class input_format_t { json, cbor, msgpack, ubjson, bson, bjdata };
 
@@ -410,7 +410,7 @@ struct container_input_adapter_factory< ContainerType,
 }
        };
 
-} // namespace container_input_adapter_factory_impl
+}  // namespace container_input_adapter_factory_impl
 
 template<typename ContainerType>
 typename container_input_adapter_factory_impl::container_input_adapter_factory<ContainerType>::adapter_type input_adapter(const ContainerType& container)
@@ -489,5 +489,6 @@ class span_input_adapter
   private:
     contiguous_bytes_input_adapter ia;
 };
+
 }  // namespace detail
-}  // namespace nlohmann
+NLOHMANN_JSON_NAMESPACE_END