Fix snowflake deserializer
Fix snowflake deserializer

Fixes snowflake deserializers (ChannelId, UserId, etc.) by switching
from a usage of `deserialize_u64` to `deserialize_any`.

Our usage of `deserialize_u64` was incorrect and the erroneous behaviour
was fixed in serde_json v1.0.8. We were essentially telling serde that
the received type was a u64, when in fact it can be multiple types
(strings, u64, or an i64 just in case).

This resulted in errors like:

```
Client error: Json(ErrorImpl { code: Message("invalid type: string
\"317727377985634305\", expected identifier"), line: 1, column: 100 })
```

Due to this, simple operations such as even connecting a client failed.

Lakelezz/serenity,zeyla/serenity,zeyla/serenity.rs,acdenisSK/serenity,Flat/serenity