Add support for std::optional<T> coroutine that allows unwrapping optional values using co_await

[lewissbaker]

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));
}

[lewissbaker]

Note that MSVC as of 2017 Update 3 would not currently be able to use such an abstraction as it currently converts from get_return_object() return-value into coroutine return-type immediately rather than waiting until after the coroutine is suspended. I am told this will be addressed in a future release.

Clang does not have this problem.

[no-more-secrets]

Note that MSVC no longer has this problem, neither does gcc. Clang, as you mentioned, traditionally didn't have this problem, but it recently broke (llvm/llvm-project#56532), hopefully they will fix it. Once that happens, all three major compilers should do the lazy implicit conversion which should then support this.