Distinct JSON enum values can sanitize to the same C++ identifier
(e.g. "foo-bar" and "foo_bar" both become `foo_bar`), producing an
invalid `enum class` with duplicate constants.

Add `unique_enum_identifiers()` which appends a numeric suffix to
later collisions while preserving enum declaration order, so the
index-to-JSON-value mapping used by the generated parser stays intact.

Also add contrib/schemagen/test_schemagen.py and wire the schemagen
Python tests plus the existing example.schema.json/test_gen.cpp example
into ctest via CMakeLists.txt.

Closes #4

@@ -170,6 +170,38 @@ if(Python3_Interpreter_FOUND)
                    ${CMAKE_CURRENT_SOURCE_DIR}/test_python_bindings.py)
 endif()
 
+# schemagen tests