deserialize_identifier: add support for all types expected by serde

The default serde deserialize derive accepts 3 types for identifiers:

- The name of the field as `str`
- The name of the field as ascii bytes
- The index of the field as u64

This PR changes deserialize_identifier to have compatibility with all of these
This is necessary for Nitrokey/fido-authenticator#57,
which needs compatibility with both the str variant and the index variant

src/de.rs

@@ -803,7 +803,13 @@ impl<'de, 'a> de::Deserializer<'de> for &'a mut Deserializer<'de> {
     where
         V: Visitor<'de>,
     {
-        self.deserialize_str(visitor)
+        let major = self.peek_major()?;
+        match major {
+            MAJOR_STR => self.deserialize_str(visitor),
+            MAJOR_POSINT => self.deserialize_u64(visitor),
+            MAJOR_BYTES => self.deserialize_bytes(visitor),
+            _ => Err(Error::DeserializeBadMajor),
+        }
     }
 
     fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value>