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/partial_emoji.py

@@ -22,7 +22,10 @@ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 DEALINGS IN THE SOFTWARE.
 """
 
+import io
+
 from .asset import Asset
+from .errors import DiscordException, InvalidArgument
 from . import utils
 
 __all__ = (
@@ -149,44 +152,83 @@ class PartialEmoji(_EmojiTag):
         return utils.snowflake_time(self.id)
 
     @property
-    def url(self):
-        """:class:`Asset`: Returns the asset of the emoji, if it is custom.
+    def url(self) -> str:
+        """:class:`str`: Returns the URL of the emoji, if it is custom.
 
-        This is equivalent to calling :meth:`url_as` with
-        the default parameters (i.e. png/gif detection).
+        If this isn't a custom emoji then an empty string is returned
         """
-        return self.url_as(format=None)
+        if self.is_unicode_emoji():
+            return ''
 
-    def url_as(self, *, format=None, static_format="png"):
-        """Returns an :class:`Asset` for the emoji's url, if it is custom.
+        fmt = 'gif' if self.animated else 'png'
+        return f'{Asset.BASE}/emojis/{self.id}.{fmt}'
 
-        The format must be one of 'webp', 'jpeg', 'jpg', 'png' or 'gif'.
-        'gif' is only valid for animated emojis.
+    async def read(self):
+        """|coro|
 
-        .. versionadded:: 1.7
+        Retrieves the content of this emoji as a :class:`bytes` object.
 
-        Parameters
-        -----------
-        format: Optional[:class:`str`]
-            The format to attempt to convert the emojis to.
-            If the format is ``None``, then it is automatically
-            detected as either 'gif' or static_format, depending on whether the
-            emoji is animated or not.
-        static_format: Optional[:class:`str`]
-            Format to attempt to convert only non-animated emoji's to.
-            Defaults to 'png'
+        .. versionadded:: 2.0
 
         Raises
-        -------
+        ------
+        DiscordException
+            There was no internal connection state.
         InvalidArgument
-            Bad image format passed to ``format`` or ``static_format``.
+            The emoji isn't custom.
+        HTTPException
+            Downloading the emoji failed.
+        NotFound
+            The emoji was deleted.
+
+        Returns
+        -------
+        :class:`bytes`
+            The content of the emoji.
+        """
+        if self._state is None:
+            raise DiscordException('Invalid state (no ConnectionState provided)')
+
+        if self.is_unicode_emoji():
+            raise InvalidArgument('PartialEmoji is not a custom emoji')
+
+        return await self._state.http.get_from_cdn(self.url)
+
+    async def save(self, fp, *, seek_begin=True):
+        """|coro|
+
+        Saves this emoji into a file-like object.
+
+        .. versionadded:: 2.0
+
+        Parameters
+        ----------
+        fp: Union[BinaryIO, :class:`os.PathLike`]
+            Same as in :meth:`Attachment.save`.
+        seek_begin: :class:`bool`
+            Same as in :meth:`Attachment.save`.
+
+        Raises
+        ------
+        DiscordException
+            There was no internal connection state.
+        HTTPException
+            Downloading the emoji failed.
+        NotFound
+            The emoji was deleted.
 
         Returns
         --------
-        :class:`Asset`
-            The resulting CDN asset.
+        :class:`int`
+            The number of bytes written.
         """
-        if self.is_unicode_emoji():
-            return Asset(self._state)
 
-        return Asset._from_emoji(self._state, self, format=format, static_format=static_format)
+        data = await self.read()
+        if isinstance(fp, io.IOBase) and fp.writable():
+            written = fp.write(data)
+            if seek_begin:
+                fp.seek(0)
+            return written
+        else:
+            with open(fp, 'wb') as f:
+                return f.write(data)