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.

src/code_generators.cpp

@@ -299,7 +299,10 @@ std::string SimpleFloatConstantGenerator::NaN(float v) const {
   return this->NaN(static_cast<double>(v));
 }
 
-std::string JavaCSharpMakeRule(const bool java, const Parser &parser,
+
+namespace {
+
+static std::string JavaCSharpMakeRule(const bool java, const Parser &parser,
                                const std::string &path,
                                const std::string &file_name) {
   const std::string file_extension = java ? ".java" : ".cs";
@@ -331,6 +334,9 @@ std::string JavaCSharpMakeRule(const bool java, const Parser &parser,
   return make_rule;
 }
 
+} // namespace
+
+
 std::string JavaMakeRule(const Parser &parser, const std::string &path,
                          const std::string &file_name) {
   return JavaCSharpMakeRule(true, parser, path, file_name);
@@ -340,12 +346,16 @@ std::string CSharpMakeRule(const Parser &parser, const std::string &path,
   return JavaCSharpMakeRule(false, parser, path, file_name);
 }
 
-std::string BinaryFileName(const Parser &parser, const std::string &path,
+namespace {
+
+static std::string BinaryFileName(const Parser &parser, const std::string &path,
                            const std::string &file_name) {
   auto ext = parser.file_extension_.length() ? parser.file_extension_ : "bin";
   return path + file_name + "." + ext;
 }
 
+} // namespace
+
 bool GenerateBinary(const Parser &parser, const std::string &path,
                     const std::string &file_name) {
   if (parser.opts.use_flexbuffers) {