Code Corniness

this kind of looks corny to me.
shell/shell.go

```go
			case "&&":
				err := s.Eval(node.Left)
				if err != nil {
					return err
				}
				right := node.Right.(*ast.SimpleCommand)
				return s.execSimpleCommand(right, os.Stdout, os.Stderr, os.Stdin)
```