[MLton] MLton.Thread.atomically

[Henry Cejtin]

Althogh certainly most of the time I would use MLton.Thread.atomically (if
we had it) I am worried about the fact that it is not going to call its
thunk argument as a tail call (of course) and that that might make it
unusable in some cases.  I guess in general nothing is as powerful as
atomicBegin/atomicEnd (or as easy to screw up).  It is all vaguely reminscent
of the Functional Programming contest on optimizing html.