Add comment support (#379)

* Add comment support

* Don't print empty comments

* Add nop instruction

* Add test and note

* Add FuncWriter trait

* Remove comment support

* Add write_preamble to FuncWriter

* Fix test

* Some changes

lib/filetests/src/runone.rs

@@ -130,7 +130,7 @@ fn run_one_test<'a>(
     // Should we run the verifier before this test?
     if !context.verified && test.needs_verifier() {
         verify_function(&func, context.flags_or_isa())
-            .map_err(|e| pretty_verifier_error(&func, isa, &e))?;
+            .map_err(|e| pretty_verifier_error(&func, isa, None, &e))?;
         context.verified = true;
     }