Upgrade rust/crates/structopt to 0.3.18 am: e7bfadf396 am: b261218390 am: 0635f12c4a am: 7718ebe33b

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/structopt/+/1436575

Change-Id: I4d151ba47f397aebf69ad6b9debcbce35aa4b3c0