Upgrade rust/crates/structopt to 0.3.20

Test: make
Change-Id: I50a53b96c652aea7c84a572a87578cf3828c00e2
diff --git a/Android.bp b/Android.bp
index 595a2f2..790b4ff 100644
--- a/Android.bp
+++ b/Android.bp
@@ -6,16 +6,12 @@
     crate_name: "structopt",
     srcs: ["src/lib.rs"],
     edition: "2018",
-    features: [
-        "default",
-    ],
+    features: ["default"],
     rustlibs: [
         "libclap",
         "liblazy_static",
     ],
-    proc_macros: [
-        "libstructopt_derive",
-    ],
+    proc_macros: ["libstructopt_derive"],
 }
 
 // dependent_library ["feature_list"]
@@ -25,14 +21,14 @@
 //   clap-2.33.3 "ansi_term,atty,color,default,strsim,suggestions,vec_map"
 //   heck-0.3.1
 //   lazy_static-1.4.0
-//   libc-0.2.77
+//   libc-0.2.80
 //   proc-macro-error-1.0.4 "default,syn,syn-error"
 //   proc-macro-error-attr-1.0.4
-//   proc-macro2-1.0.21 "default,proc-macro"
+//   proc-macro2-1.0.24 "default,proc-macro"
 //   quote-1.0.7 "default,proc-macro"
 //   strsim-0.8.0
-//   structopt-derive-0.4.11
-//   syn-1.0.41 "clone-impls,default,derive,full,parsing,printing,proc-macro,quote,visit"
+//   structopt-derive-0.4.13
+//   syn-1.0.48 "clone-impls,default,derive,full,parsing,printing,proc-macro,quote,visit"
 //   textwrap-0.11.0
 //   unicode-segmentation-1.6.0
 //   unicode-width-0.1.8 "default"