[MLton] implement _address and _symbol

[Stephen Weeks]

> If the above is right, then what I need to do is add _address and
> _symbol as keywords to the parser, teach the type checker how to
> resolve the type of these things using the '_symbol "s": type;'

To be clear, I believe we want

  _symbol "s": ptrTy, cbTy;

Of course, the front end should check that ptrTy and cbTy are of the
right form, and should report an error if not.  See, e.g. the "invalid
type for import" checks and messages.

> In fact, _import # was singled out so early in the process, that you
> would simply neeed to adjust the lexer/parser and rename
> PrimKind.Symbol to PrimKind.Address.

Whoops.  Yeah, no primitives required.  Easy :-).