Make mzlib/etc' reprovide identity' from `racket/function'.

collects/mzlib/etc.rkt

@@ -4,6 +4,8 @@
          racket/local
          racket/bool
          racket/block
+         (only racket/function
+               identity)
          (only scheme/base
                build-string
                build-list
@@ -51,8 +53,6 @@
 
          begin-lifted)
 
-(define identity (lambda (x) x))
-
 (define (loop-until start done? next body)
   (let loop ([i start])
     (unless (done? i)