AK: Rename downcast<T> => verify_cast<T>

This makes it much clearer what this cast actually does: it will
VERIFY that the thing we're casting is a T (using is<T>()).

Userland/Libraries/LibWasm/AbstractMachine/AbstractMachine.cpp

@@ -397,7 +397,7 @@ Optional<InstantiationError> AbstractMachine::allocate_all_initial_phase(Module
 
             module_instance.exports().append(ExportInstance {
                 entry.name(),
-                move(address).downcast<FunctionAddress, TableAddress, MemoryAddress, GlobalAddress>(),
+                move(address).verify_cast<FunctionAddress, TableAddress, MemoryAddress, GlobalAddress>(),
             });
         }
     });