Get rid of raw pointer (Ptr{XSxp}) interfaces

[simonbyrne]

We are a bit cavalier about passing around raw pointers (sexp) objects, and so we don't always ensure that a reference is kept when doing ccalls: this is likely the cause of the occasional segfaults that are reported (e.g. #348, #431).

The easiest option is probably to change all functions to use RObjects directly, and define appropriate cconvert/unsafe_convert methods.