Use u64 serialization of Snowflake for non human-readable serializers

[majkrzak]

Snowflake deserialization uses serde::de::Deserializer deserialize_any method, which may not be provided by some formats and according the documentation should not be relied on. This for example causes UserID fail to serialize with bincode, effectively preventing storing it in native_db.

Currently serde seems not to support easy way to determinate if deserialize_any is supported or not. Although is_human_readable is provided for both Serializer and Deserializer traits.

This PR bypasses requirement of having deserialize_any available, by serializing and deserializing Snowflake values as u64 for non human-readable formats. This additionally optimizes its storage size for such formats.