Use anonymous namespaces for private functions in C++ files

[arcondello]

Turns out just because a function is only defined in a .cpp file, doesn't mean that it is private. E.g., I added

// flow.hpp
int namespace_experiment(int a) {
    return a;
}

and then

// test_flow.cpp
int namespace_experiment(int a);

and that works just fine despite the declaration not being in the corresponding header, but rather an unrelated one.

But adding

// flow.hpp
namespace {
int namespace_experiment(int a) {
    return a;
}
}

correctly makes it private.

We should fix this everywhere.

AI Generation Disclosure

I found this out when Claude used an anonymous namespace in #590 which prompted me to learn more about it.