api: Add go files

I know this is a huge commit, but I can't be bothered to check
this in part by part.

api/commenter_session_update_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestCommenterSessionUpdateBasics(t *testing.T) {
+	failTestOnError(t, setupTestEnv())
+
+	session, _ := commenterSessionNew()
+
+	if err := commenterSessionUpdate(session, "temp-commenter-hex"); err != nil {
+		t.Errorf("unexpected error updating session to commenterHex: %v", err)
+		return
+	}
+}
+
+func TestCommenterSessionUpdateEmpty(t *testing.T) {
+	failTestOnError(t, setupTestEnv())
+
+	if err := commenterSessionUpdate("", "temp-commenter-hex"); err == nil {
+		t.Errorf("expected error not found when updating with empty session")
+		return
+	}
+}