Horrible workaround for libc++

Libc++ appears to have a problem with dynamic_casting between types
where one of them is a template that has been forward declared. This is
the only case that I see this problem coming up.

Due to the existing interdependencies between header files, I *must*
forward declare the Assignable_Proxy_Function_Impl. I don't see any
other way around this than to work around the standard library bug.

include/chaiscript/dispatchkit/type_conversions.hpp

@@ -136,6 +136,14 @@ namespace chaiscript
                         {
                           return data;
                         } else {
+#ifdef CHAISCRIPT_LIBCPP
+                          if (std::string(typeid(To).name()).find("Assignable_Proxy_Function") != std::string::npos) {
+                            auto from = detail::Cast_Helper<std::shared_ptr<From> >::cast(t_from, nullptr);
+                            if (std::string(typeid(*from).name()).find("Assignable_Proxy_Function_Impl") != std::string::npos) {
+                              return std::static_pointer_cast<To>(from);
+                            }
+                          }
+#endif
                           throw std::bad_cast();
                         }
                       }()