Add async_generator<T> type

[lewissbaker]

Add a coroutine type that allows both co_yield and co_await.

[lewissbaker]

Initial version of the async_generator has been added in f7bfad1

It uses an atomic integer to manage the state of the generator to handle potential races between consumer and producer coroutines and so has a cost of at least one atomic compare-exchange operation per item of the sequence.

In theory, if the compilers could guarantee tail-calls to await_suspend() and coroutine_handle::resume() then we could eliminate the need for this atomic integer. However, for the time-being the atomic integer is is needed to avoid blowing the stack by a potentially unbounded mutual recursion of producer/consumer coroutines.