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_*

Makefile

@@ -15,6 +15,9 @@ SED:=$(shell command -v gsed || which sed)
 # the list of sources in the include folder
 SRCS=$(shell find include -type f | sort)
 
+# the list of sources in the tests folder
+TESTS_SRCS=$(shell find tests -type f \( -name '*.hpp' -o -name '*.cpp' -o -name '*.cu' \) -not -path 'tests/thirdparty/*' | sort)
+
 # the single header (amalgamated from the source files)
 AMALGAMATED_FILE=single_include/nlohmann/json.hpp
 
@@ -159,11 +162,11 @@ pretty:
 	    --preserve-date \
 	    --suffix=none \
 	    --formatted \
-	   $(SRCS) $(AMALGAMATED_FILE) tests/src/*.cpp tests/src/*.hpp tests/benchmarks/src/benchmarks.cpp docs/examples/*.cpp
+	   $(SRCS) $(TESTS_SRCS) $(AMALGAMATED_FILE) docs/examples/*.cpp
 
 # call the Clang-Format on all source files
 pretty_format:
-	for FILE in $(SRCS) $(AMALGAMATED_FILE) tests/src/*.cpp tests/src/*.hpp benchmarks/src/benchmarks.cpp docs/examples/*.cpp; do echo $$FILE; clang-format -i $$FILE; done
+	for FILE in $(SRCS) $(TESTS_SRCS) $(AMALGAMATED_FILE) docs/examples/*.cpp; do echo $$FILE; clang-format -i $$FILE; done
 
 # create single header file
 amalgamate: $(AMALGAMATED_FILE)