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

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

Change-Id: Ia0fa426b5c9e1480a53e7476d325b53f3e79a647