Add partial Debug impls for Context and YubiKey (#457)

This enables `yubikey::Result<T>` to be debug-formatted, for example
when wrapping the output of an API method in `dbg!()`.

src/reader.rs

@@ -4,6 +4,7 @@ use crate::{Result, YubiKey};
 use std::{
     borrow::Cow,
     ffi::CStr,
+    fmt,
     sync::{Arc, Mutex},
 };
 
@@ -19,6 +20,12 @@ pub struct Context {
     reader_names: Vec<u8>,
 }
 
+impl fmt::Debug for Context {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Context").finish_non_exhaustive()
+    }
+}
+
 impl Context {
     /// Open a PC/SC context, which can be used to enumerate available PC/SC
     /// readers (which can be used to connect to YubiKeys).

src/yubikey.rs

@@ -156,6 +156,16 @@ pub struct YubiKey {
     pub(crate) serial: Serial,
 }
 
+impl fmt::Debug for YubiKey {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("YubiKey")
+            .field("name", &self.name)
+            .field("version", &self.version)
+            .field("serial", &self.serial)
+            .finish_non_exhaustive()
+    }
+}
+
 impl YubiKey {
     /// Open a connection to a YubiKey.
     ///