Add c_str() member function for rust::String
diff --git a/include/cxx.h b/include/cxx.h
index f41dde7..fc63095 100644
--- a/include/cxx.h
+++ b/include/cxx.h
@@ -53,6 +53,8 @@
   std::size_t size() const noexcept;
   std::size_t length() const noexcept;
 
+  const char *c_str() noexcept;
+
   using iterator = char *;
   iterator begin() noexcept;
   iterator end() noexcept;