this will be the basis of non-boost coroutines in lix. anything that is
a boost coroutine *should* be representable with a Generator coroutine,
and many things that are not currently boost coroutines but behave much
like one (such as, notably, serializers) should be as well. this allows
us to greatly simplify many things that look like iteration but aren't.
Change-Id: I2cebcefa0148b631fb30df4c8cfa92167a407e34
