Consider requiring explicit indices

[robin-nitrokey]

We already learned the hard way that the automatic indexing based on the field order can be a footgun. Skipping fields as implemented in #14 makes it even easier to break compatibility with older versions.

As a workaround, we could require explicit indices (or at least provide an alternative macro that works with explicit indices), for example:

#[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
pub struct SomeKeys {
    #[serde_indexed(index = 1)]
    pub number: i32,
    #[serde(skip_serializing_if = "Option::is_none")]
    #[serde_indexed(index = 2)]
    pub option: Option<u8>,
    #[serde_indexed(index = 3)]
    pub bytes: [u8; 7],
}