Mark named_arg constructor as deleted

To avoid an implicit conversion to writable::r_vector, which caused
confusion

Author: jimhester

NEWS.md

@@ -1,5 +1,6 @@
 # cpp11 (development version)
 
+* read-only `r_vector` constructors now disallow implicit construction with named arguments (#237)
 * read-only `r_vector.attr()` methods now return const objects, so it is a compile time error to try to assign to them (#237)
 * Allow `cpp11::matrix` to be accessed either row-wise or column-wise (the default) depending on the user's choice (@alyst, #229)
 * Fixed `+` and `+=` operators of `r_vector::[const_]iterator` to conform the *iterators* concept:

inst/include/cpp11/r_vector.hpp

@@ -103,6 +103,7 @@ class r_vector {
   };
 
   r_vector(const writable::r_vector<T>& rhs) : r_vector(static_cast<SEXP>(rhs)) {}
+  r_vector(named_arg) = delete;
 
   bool is_altrep() const;