Unwrapping extptr stored in capsules

[llaniewski]

R external pointers were wrapped as a python Capsule (in r_extptr_capsule(...)) but never properly unwrapped by py_to_r. As the r_extptr_capsule(...) function stores the pointer address in the Capsule, and the corresponding R object in the Capsule's "context" field, the original R external pointer can be easily retrieved. This preserves the external pointer's finaliser.

Example of usage:

library(Rcpp)
library(reticulate)

# Mock class for testing
Rcpp::sourceCpp(code='
#include <Rcpp.h>
using namespace Rcpp;

class AClass {
public:
  AClass() {
    printf("AClass created\\n");
  }
  ~AClass() {
    printf("AClass destroyed\\n");
  }
};

// [[Rcpp::export]]
SEXP getA() {
  AClass* ptr = new AClass;
  return Rcpp::XPtr< AClass >(ptr);
}
')

x = getA() #<--- AClass created
xpy = r_to_py(x)
x_ = py_to_r(xpy)

identical(x,x_) #<--- TRUE

# Removing objects one by one with gc called to check when the C++ object is destroyed.
rm(x); for (i in 1:3) gc(full = TRUE);
rm(xpy); for (i in 1:3) gc(full = TRUE);
rm(x_); for (i in 1:3) gc(full = TRUE); #<--- AClass destroyed

This PR should not break any dependencies, as the capsule visible in python still stores the same pointer. I only changed its "name" field from NULL to "r_extptr" to easily identify it.
This PR fixes #1511

[t-kalinowski]

Very nice, Thank you!