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.

src/model/mod.rs

@@ -67,7 +67,7 @@ macro_rules! id_u64 {
                     self
                 }
             }
-            
+
             impl From<u64> for $name {
                 fn from(id_as_u64: u64) -> $name {
                     $name(id_as_u64)
@@ -94,7 +94,7 @@ macro_rules! id_u64 {
 
             impl<'de> Deserialize<'de> for $name {
                 fn deserialize<D: Deserializer<'de>>(deserializer: D) -> StdResult<Self, D::Error> {
-                    deserializer.deserialize_u64(U64Visitor).map($name)
+                    deserializer.deserialize_any(U64Visitor).map($name)
                 }
             }
         )*