Fix to_string to call cpp2::to_string

Calling std::to_string makes it impossible to use to_string
on optional that contains std::string.

The solution is to call cpp2::to_string instead of std::to_string.

Author: filipsajdak

include/cpp2util.h

@@ -741,7 +741,7 @@ auto to_string(std::string const& s) -> std::string const&
 template<typename T>
 auto to_string(std::optional<T> const& o) -> std::string {
     if (o.has_value()) {
-        return std::to_string(o.value());
+        return cpp2::to_string(o.value());
     }
     return "(empty)";
 }