Register function with namespace

[nbenn]

When registering a function in a namespace such as

[[cpp11::register]] int foo::bar(int a) {
  return a;
}

decor recognizes the function name as foo::bar and all would be fine (as far as I can tell), if the generated extern "C" wrapper

cpp11/R/register.R

Lines 171 to 176 in c22dc9b

	{return_type} {name}({real_params});
	extern "C" SEXP _{package}_{name}({sexp_params}) {{
	BEGIN_CPP11
	{calls}
	END_CPP11
	}}

would not blindly forward the ::. Some further minor adjustments are required for the generation of the R-side functions. For the R-side function name, we could for example strip away anything that comes before the last scope resolution operator.

I started putting something together under main...nbenn:main and I'm happy to clean this up a bit, add some tests and submit a PR.