Calling r functions in extension packages

[andy-thomason]

Allow more flexible expressions in call! macros and update the Function
wrapper to accept primitives.

[clauswilke]

Generally looks good to me. I had a few minor comments and questions, see specific comments.

[andy-thomason]

There are many ways we can optimise here when the time comes.

We could even generate stubs for R libraries at compile time with something like:

mod base {
   include_R_namespace!("base");
}

mod stats {
   include_R_namespace!("stats");
}


...

base::sum(...);
rstats::dbeta(...);

[clauswilke]

Great, thanks! Just saw one more minor typo.

[andy-thomason]

If there are no objections, I'll merge.

[clauswilke]

Yes, go ahead.