[racket] `immutable?` for `struct`s?

Thank you for the great explanation.

Requiring #:transparent is fine for my purposes.

Although the fact that structs can't be equal? without #:transparent
is surprising to newcomers (at least it was surprising to me), that's
how structs work in Racket. My goal is to make "egal?" behave like
equal? for immutable structs, and it will wrt #:transparent.

Thanks again.