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/iterators/iter_impl.hpp

@@ -19,10 +19,10 @@
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
-namespace nlohmann
-{
+NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
+
 // forward declare, to be able to friend it later on
 template<typename IteratorType> class iteration_proxy;
 template<typename IteratorType> class iteration_proxy_value;
@@ -746,5 +746,6 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     /// the actual iterator of the associated instance
     internal_iterator<typename std::remove_const<BasicJsonType>::type> m_it {};
 };
-} // namespace detail
-} // namespace nlohmann
+
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END