[MLton] MLton.TextIO.mkstemps

[Henry Cejtin]

Something that will make the problem (with the assembler unlinking the file)
MUCH less likely to cause a problem is to include the process ID as part
of the file name (as part of the random bytes).  That way collisions are
much less common (even though the race condition still exists).

I was looking at the code trying to make sure that we (MLton) was never
incorrectly unlinking the file until all uses were done.  I didn't see any,
but it wasn't completely clear to me.