Fix bug in expander

When using `make-rule-parser` for any rule that is not the start
rule, and applying the resulting parser while giving it a source-path
it would just use the entire grammar from the start rule.

brag-lib/brag/codegen/expander.rkt

@@ -78,15 +78,15 @@
                       (member (syntax-e #'start-rule) 'RULE-IDS))
                  ;; The cfg-parser depends on the start-rule provided in (start ...) to have the same
                  ;; context as the rest of this body. Hence RECOLORED-START-RULE
-                 (with-syntax ([RECOLORED-START-RULE (datum->syntax #'RULES-STX (syntax-e #'start-rule))])
-                   #'(let ([THE-GRAMMAR (cfg-parser (tokens enumerated-tokens)
+                 (with-syntax ([RECOLORED-START-RULE (datum->syntax #'RULES-STX (syntax-e #'start-rule))]
+                               [PARSE-NAME (datum->syntax #'RULES-STX (string->symbol (format "~a-rule-parser" 'start-rule)))])
+                   #'(let* ([THE-GRAMMAR (cfg-parser (tokens enumerated-tokens)
                                                      (src-pos)
                                                      (start RECOLORED-START-RULE)
                                                      (end EOF)
                                                      (error THE-ERROR-HANDLER)
-                                                    (grammar . GENERATED-RULE-CODES))])
-                       (procedure-rename
-                        (case-lambda [(tokenizer)
+                                                     (grammar . GENERATED-RULE-CODES))]
+                            [THE-BODY (lambda (tokenizer) 
                                         (define next-token
                                           (make-permissive-tokenizer tokenizer all-tokens-hash/mutable))
                                         ;; little post-processor to support cuts on top rule name
@@ -97,10 +97,13 @@
                                         (if (eq? (syntax-property top-rule-name-stx 'hide-or-splice?) 'hide)
                                             ;; use `remove-rule-name` so we get the same housekeeping
                                             (remove-rule-name parse-tree-stx) 
-                                          parse-tree-stx)]
+                                            parse-tree-stx))])
+                       (procedure-rename
+                        (case-lambda [(tokenizer)
+                                      (THE-BODY tokenizer)]
                                      [(source tokenizer)
                                       (parameterize ([current-source source])
-                                        (PARSE tokenizer))])
+                                        (THE-BODY tokenizer))])
                         (string->symbol (format "~a-rule-parser" 'start-rule)))))]
                 [(_ not-a-rule-id)
                  (raise-syntax-error #f