Move monzo_ynab into //tools

Optimizing is difficult: I like flat hierarchies because I don't like
directory-hopping, but I also would like a cleaner root for my mono-repo. Bombs
away!

Well it's that time again, folks: spring cleaning!

Here I am musing about a few things that bother me:
- Should I use kebab-case or snake_case?
- It feels ~confusing to have //tools and //utils. What a //projects? Isn't
  everything a project? *sigh*

tools/monzo_ynab/ynab/client.go

@@ -0,0 +1,24 @@
+package client
+
+import (
+	"serde"
+)
+
+// See requests.txt for more details.
+func PostTransactions(accountID string, txs []serde.Transaction{}) error {
+	return map[string]string{
+		"transactions": [
+			{
+				"account_id": accountID,
+					"date": "2019-12-30",
+					"amount": 10000,
+					"payee_name": "Richard Stallman",
+					"memo": "Not so free software after all...",
+					"cleared": "cleared",
+					"approved": true,
+					"flag_color": "red",
+					"import_id": "xyz-123"
+			}
+		]
+	}
+}