Expose the mk-rhs function normalize-definition uses.

I found I wanted this to make a define/stub macro that errors giving the defined identifier:

(define-syntax (define/stub stx)
  (syntax-case stx ()
    [(_ header)
     (let-values ([(id mk-rhs body) (normalize-definition/mk-rhs stx #'lambda #t #t #f)])
       #`(define #,id #,(mk-rhs #`(error '#,id "TODO: stub"))))]))