if no future thread is running the future; also adjust the
policy for suspending a future so that even synchronized
operations can suspend if there's other work to be done;
also also fix `current-future' for nested `touch'es and when
parallel futures are disabled
which required adding a notion of "lightweight continuation" to
the runtime system, where a lightweight continuation involves
only frames from JIT0generated code (so that details of the stack
layout are known, for example)
