* module/ice-9/futures.scm (%futures-waiting, %within-future?,
%future-prompt): New variables.
(let/ec): New macro.
(process-future!): Run FUTURE's thunk in a prompt; capture FUTURE's
continuation when it aborts, and add it to %FUTURES-WAITING. Set
%WITHIN-FUTURE? in the dynamic extent of the call FUTURE's thunk.
(process-futures): Move loop body to...
(process-one-future): ... here. New procedure.
(notify-completion): New procedure.
(touch)[work, loop]: New procedures.
When %WITHIN-FUTURE? and FUTURE is started, abort; if not
%WITHIN-FUTURE, call `work' while waiting.
When FUTURE is queued, call `work' too.
* test-suite/tests/future.test ("nested futures"): New tests.
* module/ice-9/futures.scm (process-future!): Use `call-with-values'
when invoking `(future-thunk future)'.
* test-suite/tests/future.test ("futures")["multiple values"]: New test.
