FR066 09.05.4 Make unhandled_exception in promise types optional

[wg21bot]

Defining a coroutine’s promise type forces people who do not care about exceptions to write more boilerplate than necessary.

Proposed change:
std::coroutine_traits<....>::promise_type::unhandled_exception should be optional and equivalent to exception re-throw when not declared.

[jfbastien]

Will be addressed by resolution to #61.

[jensmaurer]

Duplicate of #61