Clean up the public interface #51
Description
The last few PRs have made some unfortunate changes to the overall API. I merged #45 because the changes to Header are good, but factoring out ContextBase was a mistake. (For example, it's silly to track keys in one place and counters in another.) In #47, we added the custom non-allocating vector and map types, which should not be externally visible (cf. #50). We should clean this up, and make sure that sframe.h is as small and readable as possible. Even if we don't have doc generation (#1), we could at least have some more extensive comments.