External pointers and attributes about cpp11

[stephematician]

In addition - there's some strange behaviour/errors that I couldn't pin down while moving duplicated external pointers around, e.g.

#include "cpp11.hpp"

using namespace cpp11;

[[cpp11::register]]
void cpp_death() {
    external_pointer<int> value_xptr(new int [8]);
    external_pointer<int> duplicate_xptr(value_xptr);
    value_xptr = std::move(duplicate_xptr);
}

It could be because external_pointer::r_deleter is called on the old_data (in external_pointer::reset) regardless of whether it should be called or not (I would have thought that r_deleter calls were managed by R when it finalised an EXTPTRSXP).