FStar.IO could do with a flush

[briangmilnes]

For testing, I write log files then read them and do a string compare to the expected. Useful for things like running external but small tests. It would be good to have the write log flushed before reading.

FStar.Unix the new wrapper I've finished (but not merged) has flush of course. Should we make a flush in FStar.IO so that we can call it in fstar/rst/fsharp/c?