Switch to using serde for deserialization

The current build system is rudimentary, incomplete, and rigid, offering
little in the way of customizing decoding options.

To solve this, switch to using serde-derive with custom Deserialization
implementations. This allows very simple deserialization when special
logic does not need to be applied, yet allows us to implement our own
deserialization logic when required.

The problem with the build system was that it built enums and structs
from YAML files. This is not so good, because it requires creating a
custom build system (which was rudimentary), creating "special struct
configs" when logic needed to be ever so slightly extended (rigid), and
if special logic needed to be applied, a custom deserialization method
would have been needed to be made anyway (incomplete).

To solve this, switch to serde-derive and implementing Deserialize
ourselves where required. This reduces YAML definitions that might
look like:

```yaml
---
name: Group
description: >
  A group channel, potentially including other users, separate from a [`Guild`].
    [`Guild`]: struct.Guild.html
fields:
  - name: channel_id
    description: The Id of the group channel.
    from: id
    type: ChannelId
  - name: icon
    description: The optional icon of the group channel.
    optional: true
    type: string
  - name: last_message_id
    description: The Id of the last message sent.
    optional: true
    type: MessageId
  - name: last_pin_timestamp
    description: Timestamp of the latest pinned message.
    optional: true
    type: string
  - name: name
    description: The name of the group channel.
    optional: true
    type: string
  - name: owner_id
    description: The Id of the group channel creator.
    type: UserId
  - name: recipients
    description: Group channel's members.
    custom: decode_users
    t: UserId, Arc<RwLock<User>>
    type: hashmap
```

to:

```rs
/// A group channel - potentially including other [`User`]s - separate from a
/// [`Guild`].
///
/// [`Guild`]: struct.Guild.html
/// [`User`]: struct.User.html
pub struct Group {
    /// The Id of the group channel.
    #[serde(rename="id")]
    pub channel_id: ChannelId,
    /// The optional icon of the group channel.
    pub icon: Option<String>,
    /// The Id of the last message sent.
    pub last_message_id: Option<MessageId>,
    /// Timestamp of the latest pinned message.
    pub last_pin_timestamp: Option<String>,
    /// The name of the group channel.
    pub name: Option<String>,
    /// The Id of the group owner.
    pub owner_id: UserId,
    /// A map of the group's recipients.
    #[serde(deserialize_with="deserialize_users")]
    pub recipients: HashMap<UserId, Arc<RwLock<User>>>,
}
```

This is much simpler and does not have as much boilerplate.

There should not be any backwards incompatible changes other than the
old, public - yet undocumented (and hidden from documentation) - decode
methods being removed. Due to the nature of this commit, field names may
be incorrect, and will need to be corrected as deserialization errors
are found.

1 files changed, 37 insertions, 6 deletions

diff --git a/src/model/permissions.rs b/src/model/permissions.rs
index b3345eb..09fad4a 100644
--- a/src/model/permissions.rs
+++ b/src/model/permissions.rs
@@ -37,7 +37,10 @@
 //! [Manage Roles]: constant.MANAGE_ROLES.html
 //! [Manage Webhooks]: constant.MANAGE_WEBHOOKS.html
 
-use ::internal::prelude::*;
+use serde::de::{Error as DeError, Visitor};
+use serde::{Deserialize, Deserializer};
+use std::fmt::{Formatter, Result as FmtResult};
+use std::result::Result as StdResult;
 
 /// Returns a set of permissions with the original @everyone permissions set
 /// to true.
@@ -145,6 +148,7 @@ pub fn voice() -> Permissions {
 
     CONNECT | SPEAK | USE_VAD
 }
+
 bitflags! {
     /// A set of permissions that can be assigned to [`User`]s and [`Role`]s via
     /// [`PermissionOverwrite`]s, roles globally in a [`Guild`], and to
@@ -253,11 +257,6 @@ bitflags! {
 }
 
 impl Permissions {
-    #[doc(hidden)]
-    pub fn decode(value: Value) -> Result<Permissions> {
-        Ok(Self::from_bits_truncate(value.as_u64().unwrap()))
-    }
-
     /// Shorthand for checking that the set of permissions contains the
     /// [Add Reactions] permission.
     ///
@@ -482,3 +481,35 @@ impl Permissions {
         self.contains(self::USE_VAD)
     }
 }
+
+impl Deserialize for Permissions {
+    fn deserialize<D: Deserializer>(deserializer: D) -> StdResult<Self, D::Error> {
+        Ok(Permissions::from_bits_truncate(deserializer.deserialize_u64(U64Visitor)?))
+    }
+}
+
+struct U64Visitor;
+
+impl Visitor for U64Visitor {
+    type Value = u64;
+
+    fn expecting(&self, formatter: &mut Formatter) -> FmtResult {
+        formatter.write_str("an unsigned 64-bit integer")
+    }
+
+    fn visit_i32<E: DeError>(self, value: i32) -> StdResult<u64, E> {
+        Ok(value as u64)
+    }
+
+    fn visit_i64<E: DeError>(self, value: i64) -> StdResult<u64, E> {
+        Ok(value as u64)
+    }
+
+    fn visit_u32<E: DeError>(self, value: u32) -> StdResult<u64, E> {
+        Ok(value as u64)
+    }
+
+    fn visit_u64<E: DeError>(self, value: u64) -> StdResult<u64, E> {
+        Ok(value)
+    }
+}