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/message.py

@@ -172,11 +172,11 @@ class Attachment(Hashable):
             The number of bytes written.
         """
         data = await self.read(use_cached=use_cached)
-        if isinstance(fp, io.IOBase) and fp.writable():
+        if isinstance(fp, io.RawIOBase):
             written = fp.write(data)
             if seek_begin:
                 fp.seek(0)
-            return written
+            return written or 0
         else:
             with open(fp, 'wb') as f:
                 return f.write(data)