Clean up anonymous namespace usage #1006
Description
Description
An anonymous namespace in C++ makes its contents global at the translation unit level (though it ensures that their symbols won't conflict with those of any other anonymous namespaces from other TUs at link time). I think it might be safe to say we have no use for these in Stan Math, and that likely the existing uses were thinking that they were file-local instead of TU-local.
Expected Output
We should refactor these to be namespace internal { instead of namespace {.
Current Math Version
v2.18.0