2021 edition upgrade; MSRV 1.56 (#343)

Changes the `edition` to 2021 in both the `yubikey` and `yubikey-cli`
crates.

Removes `TryFrom`/`TryInto` imports, now that they're in the prelude.

src/reader.rs

@@ -3,7 +3,6 @@
 use crate::{Result, YubiKey};
 use std::{
     borrow::Cow,
-    convert::TryInto,
     ffi::CStr,
     sync::{Arc, Mutex},
 };
@@ -45,6 +44,7 @@ impl Context {
             c.list_readers(reader_names)?.collect()
         };
 
+        #[allow(clippy::needless_collect)]
         let readers: Vec<_> = reader_cstrs
             .iter()
             .map(|name| Reader::new(name, Arc::clone(ctx)))