Figure out namespaces

[sakrejda]

Looks like:

• [ ] stan::proto should be where the message types live, we got this right.
• [ ] stan::interface_callbacks::writer is where the new writers should go, even though internally they do some protobuf stuff, they're still writers and should live with all the writers.
• [ ] stan::proto::compile is in stan::proto, rather than stan::lang, even though it's a "compile"

Looks like we are exposing three types of things: 1) messages; 2) thin wrappers; and 3) callbacks. So generalizing to a namespace policy:

• If it's a message it lives in stan::proto
• If it's takes message arguments and returns messages, it's part of the thin wrapper and should also live in stan::proto
• If it's a callback it's implementing additional logic and not necessarily dealing with protobuf types as function inputs/outputs so it goes in whatever namespace that type of callback usually goes.

@ariddell Feel free to comment, close if you agree and I'll write it up as a Wiki thing and/or add to README.md

[ariddell]

I don't know the C++ convention here. Does one try to keep an external package in its own namespace? For example, what if by accident we end up clobbering a function name (say, 'compile') in stan::lang with the protostan version?

[sakrejda]

That should only be an issue for callbacks in the scheme outlined above and there... I don't know. At least callbacks are pretty job-specific and tend to have less common names (and more specialized purposes) so I don't see this being a big issue.

[ariddell]

Anyway, this all sounds good.