Audit and fixups for GCC and Clang (#7212)

Added (for compiler versions that support it):
-Wmissing-declarations
-Wzero-as-null-pointer-constant

Then, fixes to problems identified by the extra warnings
Tested only on GCC 9.4.0

Adjusted the CPP code generator to output nullptr where appropriate,
to satisfy -Wzero-as-null-pointer-constant

Added a lot of 'static' declarations in front of functions,
to satisfy -Wmissing-declarations,
and wrap static function defs in anonymous namespaces.

There are advantages to both anonymous namespaces and static,
it seems that marking a function as static will not publish the name in
the symbol table at all, thus giving the linker less work to do.

tests/fuzzer/test_init.h

@@ -11,7 +11,7 @@ struct OneTimeTestInit {
   // This hook terminate program both in Debug and Release.
   static bool TestFailListener(const char *expval, const char *val,
                                const char *exp, const char *file, int line,
-                               const char *func = 0) {
+                               const char *func = nullptr) {
     (void)expval;
     (void)val;
     (void)exp;