Function string_views

test/generator_test.cpp:142–145

value_type = std::string_view reference = std::string_view&&

// value_type = std::string_view
// reference = std::string_view&&
co_context::generator<std::string_view> string_views() {
    co_yield "foo";
    co_yield "bar";
}

// value_type = std::string
// reference = std::string_view