minor optimization: don't wrap a 'begin' that contains a single expression

svn: r2877

collects/errortrace/errortrace-lib.ss

@@ -305,7 +305,7 @@
                             e
                             (namespace-syntax-introduce
                              (datum->syntax-object #f e))))])
-               e2)
+	       e2)
              e)
          immediate-eval?))))
 

collects/errortrace/stacktrace.ss

@@ -473,6 +473,11 @@
                             (rebuild expr (list (cons #'rhs new-rhs))))))]
 
              ;; Wrap subexpressions only
+	     [(begin e)
+	      ;; Single expression: no mark
+	      (certify
+	       expr
+	       #`(begin #,(annotate (syntax e) trans?)))]
              [(begin . body)
               (with-mark expr
                          (certify