We make use of constexpr to handle the case where we need to co_await the
callback and when we just want to return the result of the callback without
co_awaiting it.
For the optional ErrorCallback we simply provide a default one which just
rethrows the current exception.

Use constexpr to handle invocation with arguments or without.

Avoid confusion with std::invoke and std::invoke_result. The new
names refer to callbacks.