Get tests passing again by returning keyword rather than nil from chunks

src/boot/boot.janet

@@ -2017,7 +2017,8 @@
   (while going
     (if (env :exit) (break))
     (buffer/clear buf)
-    (if (nil? (chunks buf p))
+    (if (= (chunks buf p)
+           :cancel)
       (do
         # Nil chunks represents a cancelled form in the REPL, so reset.
         (parser/flush p)

src/mainclient/shell.c

@@ -63,7 +63,8 @@ Janet janet_line_getter(int32_t argc, Janet *argv) {
         gbl_cancel_current_repl_form = false;
 
         // Signal that the user bailed out of the current form
-        result = janet_wrap_nil();
+        const static char *const msg = "cancel";
+        result = janet_ckeywordv(msg);
     } else {
         result = janet_wrap_buffer(buf);
     }