Multiple timeout values

[shirok]

We have APIs that takes timeout and timeout-val, e.g. future-get and with-timeout. They allow main expression to return MVs, so it is natural that it can also return MVs for the timeout-val. How shall we let the caller specify that?

• We can use rest arguments as timeout-vals, e.g. (future-get expr timeout val-a val-b val-c). This is backward-compatible. Prevents future extension of futher optional arguments.
• Add a new API that allows multiple timeout-vals, e.g. a thunk that's called when timeout occurs. (future-get/timeout-thunk expr timeout (lambda () (values val-a val-b val-c))). The name is debatable.

(Stemmed from #1103 (comment) )