add source line number preservation

For source-level debug. Add --line-directives option, defaulted to
statement. Disabled when used with -C. Explicit non-none value
overrides disabling when -C is present

tools/sextest/utils.module.scm

@@ -5,8 +5,13 @@
      list-split
      list-join
      recons
-     set-form-line!
+     current-source-file
+     set-form-source!
+     form-source
+     form-file
      form-line
+     copy-form-source!
+     stamp-form-source!
      with-directory
      )
   "../../utils.scm")