Return as variable

Dyon allows you to assign a value to `return`, as if it was a normal local variable:

``` rust
fn foo() -> {
     return = 4
     return = 5
     // will return 5
}
```

This requires `->` on the function.

The `return` value outlives the arguments of the functions, unless they have a lifetime. If you assign to it, the right expression must outlive `'return`.
