Implement wrappers for function-like macros

I'm interested in accessing some reference counting functionality in R, but I noticed that e.g. `MAYBE_SHARED`, `NO_REFERENCES` have no bindings generated. This is of course because of the fact that bindgen can't (yet) generate macros for function-like macros: https://github.com/rust-lang/rust-bindgen/issues/753.

Therefore I'm thinking it might be nice to slowly add some *manual* implementations of these macros. The simple ones like `#define CHAR(x) R_CHAR(x)` are of lower priority to me since they don't represent missing functionality, but stuff like `# define MAYBE_SHARED(x) (NAMED(x) > 1)` seems useful to me, as are more complex ones like `KNOWN_SORTED`.

The only question is whether we want these to live in `libR-sys` as opposed to `extendr`, since they're not generated and would be the first manual functions in the crate. Opinions are welcome.