Remove most uses of mzscheme in the core.

Remaining are:
 - parts of unit200 that Matthew plans to remove.
 - the `mzscheme` implementation itself.

The implementation of `mzscheme` has been moved
to the `mzscheme` collection (from the `racket` and
`scheme` collections). The `scheme/mzscheme`
language, which was undocumented, has been removed.

This is slightly backwards-incompatible, because
the `xform` handling of precompiled headers now
evaluates code in a `racket/base`-like namespace,
instead of in a `mzscheme`-like namespace.

original commit: d54c1e4e49

racket/lib/collects/compiler/bundle-dist.rkt

@@ -1,7 +1,7 @@
 
-(module bundle-dist mzscheme
+(module bundle-dist racket/base
   (require racket/file
-           (only racket/base lambda)
+           (only-in racket/base lambda)
            racket/path
 	   racket/system
 	   file/zip
@@ -80,8 +80,8 @@
 			 "-mode" "555" 
 			 "-volname" (path->string
 				     (path-replace-suffix (file-name-from-path target) #""))
-			 "-srcfolder" (path->string (expand-path (path->complete-path dir)))
-			 (path->string (expand-path (path->complete-path target))))])
+			 "-srcfolder" (path->string (cleanse-path (path->complete-path dir)))
+			 (path->string (cleanse-path (path->complete-path target))))])
 		((list-ref p 4) 'wait)
 		(unless (eq? ((list-ref p 4) 'status) 'done-ok)
 		  (error 'bundle-directory

racket/lib/collects/compiler/embed-sig.rkt

@@ -1,5 +1,5 @@
 
-(module embed-sig mzscheme
+(module embed-sig racket/base
   (require racket/unit)
   (provide compiler:embed^)
 

racket/lib/collects/compiler/sig.rkt

@@ -1,5 +1,4 @@
-
-#lang mzscheme
+#lang racket/base
 
 (require racket/unit)
 

racket/lib/collects/setup/option-sig.rkt

@@ -1,5 +1,4 @@
-
-(module option-sig mzscheme
+(module option-sig scheme/base
   (require racket/unit)
   
   (provide setup-option^)