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

fmt-c-writer.module.scm

@@ -1,3 +1,3 @@
 (module fmt-c-writer (emit-c
-                      sex-fmt-current-file)
+                      sex-line-directives)
   "fmt-c-writer.scm")