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?

[briangmilnes]

OCaml unix alas does not provide an O_TMPFILE flag to openfile. So we could
do with a random, a tmpfile addition and a file delete in FStar.IO. I'll build them
if approved.