Anonymous namespaces should be internal

Please don’t use anonymous namespaces in Stan C++ code. We’ve decided that we should use stan::X::internal, etc. There’s an issue in the math lib where there are still a lot of anonymous namespace usages.

Sorry this stuff isn’t all documented in one place (or perhaps anywhere). I’m going to be working on making our developer doc and standards a lot clearer over the next year.

We discussed this previous in this topic.