Cpp11

[pachadotdev]

Hi @hadley

These minimal changes allow the user to just write functions of the form

#include <cpp11.hpp>

using namespace cpp11;

[[cpp11::register]] int hello(int x) {
    return x + 1;
}

Immediately after using use_cpp11().

With this change the user can run devtools::load_all() and hello(1) will work. Otherwise it says pkgname_hello not found.

Semi-related to r-lib/cpp11#341