A few additions to external_pointer

[thk686]

In a few cases, it would make code slightly cleaner if external_pointer defined operator sexp(). It would also be nice to expose R_SetExternalPtrProtected as I use it a lot in a current project. The only reasoning for this is to expose only cpp11 code and not a mix of cpp11 and R internal functions, so it's not very important. More stylistic I suppose. I can make those changes in a fork unless there are reasons not too.