syntax/parse: Arrange for built-in macro’s names to be preserved

The dance that syntax/parse performs to lazily load its implementation
rewrites uses of syntax/parse macros in such a way that their original
names were discarded, which shows up in error messages. By simply
invoking the underlying transformer directly in the proxy macro instead
of expanding to a rewritten use, users’ names can be preserved.

fixes #1909

parse/private/sc.rkt

@@ -6,10 +6,7 @@
 ;; keep and keep as abs. path -- lazy-loaded macros produce references to this
 ;; must be required via *absolute module path* from any disappearing module
 ;; (so for consistency etc, require absolutely from all modules)
-(require stxparse-info/parse/private/residual
-         racket/syntax
-         racket/stxparam
-         syntax/stx)
+(require stxparse-info/parse/private/residual)
 
 (begin-for-syntax
  (lazy-require
@@ -60,9 +57,7 @@
                   define-eh-alternative-set)
   (let ([tx (lambda (get-id)
               (lambda (stx)
-                (syntax-case stx ()
-                  [(_ . args)
-                   (datum->syntax stx (cons (get-id) #'args) stx)])))])
+                ((syntax-local-value (get-id)) stx)))])
     (values 
      (tx id:define-syntax-class)
      (tx id:define-splicing-syntax-class)