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

cranelift/filetests/isa/x86/nop.cton

@@ -0,0 +1,10 @@
+test compile
+
+target x86_64
+
+function %test(i32) -> i32 system_v {
+ebb0(v0: i32):
+    nop
+    v1 = iconst.i32 42
+    return v1
+}