Rewrite Asset design

This is a breaking change.

This does the following transformations, assuming `asset` represents
an asset type.

Object.is_asset_animated() => Object.asset.is_animated()
Object.asset => Object.asset.key
Object.asset_url => Object.asset_url
Object.asset_url_as => Object.asset.replace(...)

Since the asset type now requires a key (or hash, if you will),
Emoji had to be flattened similar to how Attachment was done since
these assets are keyed solely ID.

Emoji.url (Asset) => Emoji.url (str)
Emoji.url_as => removed
Emoji.url.read => Emoji.read
Emoji.url.save => Emoji.save

This transformation was also done to PartialEmoji.

discord/team.py

@@ -42,8 +42,6 @@ class Team:
         The team ID.
     name: :class:`str`
         The team name
-    icon: Optional[:class:`str`]
-        The icon hash, if it exists.
     owner_id: :class:`int`
         The team's owner ID.
     members: List[:class:`TeamMember`]
@@ -52,14 +50,14 @@ class Team:
         .. versionadded:: 1.3
     """
 
-    __slots__ = ('_state', 'id', 'name', 'icon', 'owner_id', 'members')
+    __slots__ = ('_state', 'id', 'name', '_icon', 'owner_id', 'members')
 
     def __init__(self, state, data):
         self._state = state
 
-        self.id = utils._get_as_snowflake(data, 'id')
+        self.id = int(data['id'])
         self.name = data['name']
-        self.icon = data['icon']
+        self._icon = data['icon']
         self.owner_id = utils._get_as_snowflake(data, 'owner_user_id')
         self.members = [TeamMember(self, self._state, member) for member in data['members']]
 
@@ -67,40 +65,11 @@ class Team:
         return f'<{self.__class__.__name__} id={self.id} name={self.name}>'
 
     @property
-    def icon_url(self):
-        """:class:`.Asset`: Retrieves the team's icon asset.
-
-        This is equivalent to calling :meth:`icon_url_as` with
-        the default parameters ('webp' format and a size of 1024).
-        """
-        return self.icon_url_as()
-
-    def icon_url_as(self, *, format='webp', size=1024):
-        """Returns an :class:`Asset` for the icon the team has.
-
-        The format must be one of 'webp', 'jpeg', 'jpg' or 'png'.
-        The size must be a power of 2 between 16 and 4096.
-
-        .. versionadded:: 2.0
-
-        Parameters
-        -----------
-        format: :class:`str`
-            The format to attempt to convert the icon to. Defaults to 'webp'.
-        size: :class:`int`
-            The size of the image to display.
-
-        Raises
-        ------
-        InvalidArgument
-            Bad image format passed to ``format`` or invalid ``size``.
-
-        Returns
-        --------
-        :class:`Asset`
-            The resulting CDN asset.
-        """
-        return Asset._from_icon(self._state, self, 'team', format=format, size=size)
+    def icon(self):
+        """Optional[:class:`.Asset`]: Retrieves the team's icon asset, if any."""
+        if self._icon is None:
+            return None
+        return Asset._from_icon(self._state, self.id, self._icon, path='team')
 
     @property
     def owner(self):