Various problems calling ToStr on pointer types in rusti

```
rusti> let f = @1;
rusti> f.to_str()
~"1"
```

So far so good.

```
rusti> let g = (@1, @2);
rusti> g.to_str()
error: failed to find am implementation of trait std::to_str::ToStr for @int
```

Seems like this should work out...

```
rusti> let h = (~[1], ~[2]);
rusti> h.to_str()
Segmentation fault: 11
```

Running on OSX 10.8 x86_64.
