build: support comments in functions (#41)

tests/fmt/asm.go

@@ -0,0 +1,26 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/reg"
+)
+
+func main() {
+	TEXT("Formatting", NOSPLIT, "func()")
+	Doc("Formatting contains various cases to test the formatter.")
+
+	ADDQ(R8, R8)
+	Comment("One comment line between instructions.")
+	ADDQ(R8, R8)
+
+	Comment("Comment before label.")
+	Label("label")
+	Comment("Comment after label.")
+	ADDQ(R8, R8)
+
+	RET()
+
+	Generate()
+}