Unpacking `ref` field in `cases` sometimes doesn't deref

[tdelv]

Possibly an extension of #1602. Code: https://code.pyret.org/editor#share=1ML0EdMGdi5Z0iBJ0IMUg9-u6VQ4e1tR2&v=1904b2c

If the inner-most body is a cases, then we get that b is a ref on line 19, but if the body is a value or function call, it works fine.