implement ripemd160 just so we can validate peter todd's old examples.

unused.go

@@ -0,0 +1,9 @@
+package opentimestamps
+
+import (
+	deprecated_ripemd160 "golang.org/x/crypto/ripemd160"
+)
+
+func ripemd160(curr []byte, arg []byte) []byte {
+	return deprecated_ripemd160.New().Sum(curr)
+}