Restore CONS evaluation on REPL

Now this is a tricky one.

CONS evaluates to error on REPL, too:
```lisp
CL-USER> (cons 1 2)
Error occurred
```
That's because JavaScript EVAL interprets curly braces of object literals as
markers of a block of code. We can do this little trick by emitting an extra
pair of parens around object literals during code generation to avoid this.