improved expansion of -> contract combinator to generate less stuff and got a very slight performance improvement. also split up contract implementation into multiple files

svn: r1956

original commit: edc4b96987ebfa1016801536f200fe3d7056ee4a

collects/mzlib/contract.ss

@@ -1,7 +1,15 @@
 (module contract mzscheme
-  (require "private/contract.ss")
-  (provide (all-from-except "private/contract.ss"
-                            make-contract
-                            contract-proc
-                            raise-contract-error
-                            build-compound-type-name)))
+  (require "private/contract.ss"
+           "private/contract-arrow.ss"
+           "private/contract-util.ss")
+  
+  
+  (provide 
+   (all-from "private/contract-arrow.ss")
+   (all-from-except "private/contract-util.ss"
+                    raise-contract-error
+                    contract-proc
+                    make-contract
+                    contract-proc
+                    build-compound-type-name)
+   (all-from-except "private/contract.ss")))