Newtypes

This isn't ready for merging just yet, it seems there may be a problem with the optimiser that this causes or uncovers relating to TCO.

The `NewtypeTCO` test is identical to the existing `Rank2Data` test, but uses `newtype` rather than `data` definitions, and when run, the generated code contains a reference to an undeclared variable:

```
[stdin]:125
                    zero = __tco_zero;
                           ^
ReferenceError: __tco_zero is not defined
    at [stdin]:125:28
    at [stdin]:149:37
    at PS.Main.runNat ([stdin]:134:22)
    at [stdin]:155:22
    at [stdin]:170:3
    at Object.<anonymous> ([stdin]-wrapper:6:22)
    at Module._compile (module.js:456:26)
    at evalScript (node.js:536:25)
    at Socket.<anonymous> (node.js:154:11)
    at Socket.EventEmitter.emit (events.js:117:20)
```

Which we can see here:

``` js
var succ = function (__copy_n) {
    return function (__copy_zero) {
        return function (__copy_succ) {
            var n = __copy_n;
            var zero = __copy_zero;
            var succ = __copy_succ;
            tco: while (true) {
                var __tco_n = n(zero)(succ);
                n = __tco_n;
                zero = __tco_zero;
                succ = __tco_succ;
                continue tco;
            };
        };
    };
};
```

So I'm not quite sure where the `var` declaration for `__tco_zero` has gone, perhaps it's an interaction between different optimisation passes.

Here's the original definition of `succ`:

``` haskell
  succ = \n -> case n of
    Nat f -> Nat (\zero succ -> succ (f zero succ))
```
Yeah, I don't see why not. Where would we do the check/throw the error, in the typechecker?