Helper function to obtain an object's associated executor.
template< typename T, typename ExecutionContext> associated_executor< T, typename ExecutionContext::executor_type >::type get_associated_executor( const T & t, ExecutionContext & ctx, typename constraint< is_convertible< ExecutionContext &, execution_context & >::value >::type = 0);
associated_executor<T, typename
ExecutionContext::executor_type>get(t, ctx.get_executor())