add datum-case', etc. as syntax/datum'

This library is used by Redex, which wants a `syntax'-like template
language, but for datum values instead of syntax objects. Using
`datum-case' and `datum' generates much less code. Redex uses
only a small part of the general functionality, so adding
`syntax/datum' could be overkill. It's implemented by generalizing
the `syntax-case' and `syntax' pattern matching and template
constructing code, though; it's not a lot of extra code, and it's
easiest to generalize completely. We may find other uses for
datum templates, too.

original commit: 25dd8727cb8aca54dc7c5cbf1814f2049234ad1d

collects/racket/private/old-ds.rkt

@@ -4,7 +4,7 @@
              (for-syntax '#%kernel
                          "stxcase-scheme.rkt"))
 
-  (#%provide define-struct let-struct datum)
+  (#%provide define-struct let-struct old-datum)
   
   (define-syntaxes (define-struct)
     (lambda (stx)
@@ -28,6 +28,6 @@
          (define-struct base (field ...))
          body1 body ...)]))
 
-  (define-syntaxes (datum)
+  (define-syntaxes (old-datum)
     (syntax-rules ()
       [(_ . any) (quote any)])))