cli: migrate from gumdrop to clap v3 (#379)

`gumdrop` was originally chosen for its minimalist set of dependencies,
but `clap` v3 has a slimmed down set of dependencies and better UX.

cli/src/commands/readers.rs

@@ -1,7 +1,7 @@
 //! List detected readers
 
 use crate::terminal::STDOUT;
-use gumdrop::Options;
+use clap::Parser;
 use std::{
     io::{self, Write},
     process::exit,
@@ -10,7 +10,7 @@ use termcolor::{ColorSpec, StandardStreamLock, WriteColor};
 use yubikey::{Context, Serial};
 
 /// The `readers` subcommand
-#[derive(Debug, Options)]
+#[derive(Debug, Parser)]
 pub struct ReadersCmd {}
 
 impl ReadersCmd {

cli/src/commands/status.rs

@@ -1,7 +1,7 @@
 //! Print device status
 
 use crate::terminal::{print_cert_info, STDOUT};
-use gumdrop::Options;
+use clap::Parser;
 use std::io::{self, Write};
 use termcolor::{ColorSpec, StandardStreamLock, WriteColor};
 use yubikey::{piv::*, YubiKey};
@@ -10,7 +10,7 @@ use yubikey::{piv::*, YubiKey};
 const NONE_STR: &str = "<none>";
 
 /// The `status` subcommand
-#[derive(Debug, Options)]
+#[derive(Debug, Parser)]
 pub struct StatusCmd {}
 
 impl StatusCmd {