Upgrade rust/crates/structopt to 0.3.18

Test: make
Change-Id: Ib5d515b0f5e9ac7fdd0f2f8e2f00ecf54dbf63ca
diff --git a/METADATA b/METADATA
index e1deda2..b6bc6ff 100644
--- a/METADATA
+++ b/METADATA
@@ -7,13 +7,13 @@
   }
   url {
     type: ARCHIVE
-    value: "https://static.crates.io/crates/structopt/structopt-0.3.14.crate"
+    value: "https://static.crates.io/crates/structopt/structopt-0.3.18.crate"
   }
-  version: "0.3.14"
+  version: "0.3.18"
   license_type: NOTICE
   last_upgrade_date {
     year: 2020
-    month: 5
-    day: 6
+    month: 9
+    day: 23
   }
 }