Fix append operation of identifiers with spaces and strange characters

The bug report #14761 is about an error in prefix-out when one of the identifiers has a space. The solution was to change a `"~s~s"` to `"~a~a"`.

I also looked for similar patterns in the Racket repository, and I found 6 similar instances in Redex.

I don't know enough about Redex to be sure that this is the correct fix. Please review this carefully.

(I'm moving this pull request from the Racket repository to here.)