Implement `while(co_await next_frame)` support

[ImmutableOctet]

Writing co_await with an empty initializer list {} or next_frame should be made to supply a true Boolean value on coroutine resume.

This would enable programming patterns like:

while(co_await next_frame)
{
    // ...
}

do
{
    // ...
}
while(co_await next_frame);