Add support for std::optional<T> coroutine that allows unwrapping optional values using co_await #43
Description
See https://github.com/toby-allsopp/coroutine_monad for inspiration.
std::optional<int> parse_int(const std::string_view& s);
std::optional<std::tuple<int, int>> parse_int_pair(const std::string_view& a, const std::string_view& b)
{
co_return std::make_tuple(co_await parse_int(a), co_await parse_int(b));
}