Assignment of optional var to non-optional value should unwrap it.

This should work:

``` go
var x ?X
x = X{...}
// x has type X now
```

Right now you're forced to do this:

``` go
var x ?X
x = X{...}
if x != nil {
    // x has type X now
}
```

But the compiler could figure out that `x` is not nil there.

Be careful with this, though:

``` go
var x ?X
x = X{...}
// x has type X now
if something {
    x = nil
}
// x should have type ?X again now
```
