eval:alts should allow escape within eval datum

`eval:alts` currently only allows escaping within the "show" datum and not the "eval" datum, but it is useful to be able to programmatically construct the eval datum too and so I think it makes sense to allow escaping there.

```racket
#lang racket/base
(require scribble/example)
(examples (eval:alts #,(add1 1) 2)) ; allowed
(examples (eval:alts 2 #,(add1 1))) ; not allowed
```