Collect all global state for R callbacks in a struct

[lionel-]

This way all state needed by the callbacks can be inspected in one place and this makes the access syntax a bit lighter.
Also removes some unnecessary mutexes for state that is only accessed from frontend callbacks by the single R thread.

Branched from #51.

[lionel-]

Following this, we could move the frontend methods to impl RMain methods, here is an example: ffb6bbf

The idea is that we'd call let main = unsafe { R_MAIN.as_mut().unwrap() }; inside R extension points and then immediately the corresponding method on main. All these methods can then be written more conventionally with state in self, and without the R lock since the extension points are called synchronously by R (via frontend methods or .Call()). We just need to make sure the unsafe dereferences of the global R_MAIN object are indeed made from the R thread to ensure safety.

[kevinushey]

LGTM!