Creating a tuple of a parameterized string leaks memory

Test case:

```
fn f[T](T x) -> tup(T) {
  ret tup(x);
}

fn main() -> () {
  f("foo");
}
```

Result:

```
[pwalton@host-4-84 ~/Source/rust/src] ./bug 
allocation 0x600000 was not freed
rt: fatal, 'leaked memory in rust main loop (1 objects)' failed, rt/memory_region.cpp:99 1 objects
```
