ng/eval: run top-level defers before exiting interpreter

with #218 merged in, we still need to have the top-level defers run before properly closing/exiting the interpreter.

Right now, we get this:
```go
$> ng
ng> defer printf("hello\n")
ng> ^D
$>  
```

whereas, it should be:
```go
$> ng
ng> defer printf("hello\n")
ng> ^D
hello
$>  
```
