[MLton] implement _address and _symbol

[Stephen Weeks]

While I'm soliciting help, Wesley, do you have any interest in
implementing _address and _symbol?  A bit harder than implement the
allowFFI annotation, but not too bad.  MLton already has the
Pointer_{get,set} primitives, so the only additional thing needed is a
primitive to support _address.  That and a little bit of front end
hacking to expand the syntax into the appropriate lambda expressions
should do it.