Move core to racket lang.

This was an easy set of modules to port-just switched the #lang form
from #lang scheme to #lang racket.

collects/frtime/core/contract.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 
 (define-syntax-rule (provide/contract* [id ctrct] ...)
   #;(provide/contract [id ctrct] ...)

collects/frtime/core/dv.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 (require "contract.rkt")
 
 (define-struct dv (vec-length next-avail-pos vec) #:mutable)

collects/frtime/core/erl.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 (require "match.rkt"
          "contract.rkt"
          #;"sema-mailbox.rkt"

collects/frtime/core/frp.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 (require (only-in mzlib/etc 
                   identity nor)
          "contract.rkt"

collects/frtime/core/heap.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 (require "dv.rkt"
          "contract.rkt")
 

collects/frtime/core/mailbox.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 (require "contract.rkt"
          "match.rkt"
          scheme/async-channel)

collects/frtime/core/match.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 
 (define-struct a-match-fail ())
 (define match-fail (make-a-match-fail))

collects/frtime/core/sema-mailbox.rkt

@@ -1,4 +1,4 @@
-#lang scheme
+#lang racket
 (require "match.rkt"
          "contract.rkt")
 

collects/frtime/gui/aux-mixin-macros.rkt

@@ -1,6 +1,6 @@
 (module aux-mixin-macros frtime
   (require "mixin-macros.rkt")
-  (require mzlib/class)
+  (require racket/class)
 
   
   ;; consider taking out setter