Make Message::nonce a serde_json::Value

Nonces can actually be almost anything - including booleans - so just
use a Value to represent it since very few users will need it.

This fixes errors like:
```
WARN:serenity::internal::ws_impl: (╯°□°）╯︵ ┻━┻ Error decoding: {"t":"MESSAGE_CREATE","s":12187872,"op":0,"d":{"type":0,"tts":false,"timestamp":"2017-06-01T01:00:00.000000+00:00","pinned":false,"nonce":"","mentions":[{"username":"redacted","id":"redacted","discriminator":"redacted","avatar":"redacted"}],"mention_roles":[],"mention_everyone":false,"id":"redacted","embeds":[],"edited_timestamp":null,"content":"redacted","channel_id":"redacted","author":{"username":"redacted","id":"redacted","discriminator":"redacted","bot":true,"avatar":"redacted"},"attachments":[]}}
ERROR:serenity::client: Shard handler received err: Json(ErrorImpl { code: Message("Unknown i64 value: "), line: 0, column: 0 })
```

and:

```
WARN:serenity::internal::ws_impl: (╯°□°）╯︵ ┻━┻ Error decoding: {"t":"MESSAGE_CREATE","s":1001192,"op":0,"d":{"type":0,"tts":false,"timestamp":"2017-06-01T01:01:01.000000+00:00","pinned":false,"nonce":true,"mentions":[],"mention_roles":[],"mention_everyone":false,"id":"redacted","embeds":[],"edited_timestamp":null,"content":"bork","channel_id":"redacted","author":{"username":"redacted","id":"redacted","discriminator":"redacted","bot":true,"avatar":"redacted"},"attachments":[]}}
ERROR:serenity::client: Shard handler received err: Json(ErrorImpl { code: Message("invalid type: boolean `true`, expected identifier"), line: 0, column: 0 })
```

src/model/channel/message.rs

@@ -1,4 +1,5 @@
 use chrono::{DateTime, FixedOffset};
+use serde_json::Value;
 use ::model::*;
 
 #[cfg(feature="cache")]
@@ -48,7 +49,8 @@ pub struct Message {
     /// Array of users mentioned in the message.
     pub mentions: Vec<User>,
     /// Non-repeating number used for ensuring message order.
-    pub nonce: Option<Snowflake>,
+    #[serde(default)]
+    pub nonce: Value,
     /// Indicator of whether the message is pinned.
     pub pinned: bool,
     /// Array of reactions performed on the message.

src/model/mod.rs

@@ -53,50 +53,6 @@ use ::utils::Colour;
 
 fn default_true() -> bool { true }
 
-macro_rules! id_i64 {
-    ($(#[$attr:meta] $name:ident;)*) => {
-        $(
-            #[$attr]
-            #[derive(Copy, Clone, Debug, Eq, Hash, PartialOrd, Ord, Serialize)]
-            #[allow(derive_hash_xor_eq)]
-            pub struct $name(pub i64);
-
-            impl $name {
-                /// Retrieves the time that the Id was created at.
-                pub fn created_at(&self) -> NaiveDateTime {
-                    let offset = (self.0 >> 22) / 1000;
-
-                    NaiveDateTime::from_timestamp(1420070400 + offset, 0)
-                }
-            }
-
-            impl From<i64> for $name {
-                fn from(v: i64) -> $name {
-                    $name(v)
-                }
-            }
-
-            impl PartialEq for $name {
-                fn eq(&self, other: &Self) -> bool {
-                    self.0 == other.0
-                }
-            }
-
-            impl PartialEq<i64> for $name {
-                fn eq(&self, u: &i64) -> bool {
-                    self.0 == *u
-                }
-            }
-
-            impl<'de> Deserialize<'de> for $name {
-                fn deserialize<D: Deserializer<'de>>(deserializer: D) -> StdResult<Self, D::Error> {
-                    deserializer.deserialize_i64(I64Visitor).map($name)
-                }
-            }
-        )*
-    };
-}
-
 macro_rules! id_u64 {
     ($(#[$attr:meta] $name:ident;)*) => {
         $(
@@ -160,11 +116,6 @@ id_u64! {
     WebhookId;
 }
 
-id_i64! {
-    /// An identifier for a general-purpose signed snowflake.
-    Snowflake;
-}
-
 /// A container for guilds.
 ///
 /// This is used to differentiate whether a guild itself can be used or whether

src/model/utils.rs

@@ -240,34 +240,3 @@ impl<'de> Visitor<'de> for U64Visitor {
         Ok(v)
     }
 }
-
-pub struct I64Visitor;
-
-impl<'de> Visitor<'de> for I64Visitor {
-    type Value = i64;
-
-    fn expecting(&self, formatter: &mut Formatter) -> FmtResult {
-        formatter.write_str("identifier")
-    }
-
-    fn visit_str<E: DeError>(self, v: &str) -> StdResult<Self::Value, E> {
-        match v.parse::<i64>() {
-            Ok(v) => Ok(v),
-            Err(_) => {
-                let mut s = String::new();
-                s.push_str("Unknown i64 value: ");
-                s.push_str(v);
-
-                Err(DeError::custom(s))
-            },
-        }
-    }
-
-    fn visit_i64<E: DeError>(self, v: i64) -> StdResult<Self::Value, E> {
-        Ok(v)
-    }
-
-    fn visit_u64<E: DeError>(self, v: u64) -> StdResult<Self::Value, E> {
-        Ok(v as i64)
-    }
-}