Redesign asset retrieval in the library.

Most assets now return a new class named `Asset`. This allows for the
assets to be consistently saved via a `save` method instead of special
casing for `Attachment`.

`AppInfo` is no longer a namedtuple it is a fully documented dataclass,
as well as having the state attached to it.

Fixes #1997

discord/ext/commands/converter.py

@@ -400,7 +400,8 @@ class PartialEmojiConverter(Converter):
             emoji_name = match.group(2)
             emoji_id = int(match.group(3))
 
-            return discord.PartialEmoji(animated=emoji_animated, name=emoji_name, id=emoji_id)
+            return discord.PartialEmoji.with_state(ctx.bot._connection, animated=emoji_animated, name=emoji_name,
+                                                   id=emoji_id)
 
         raise BadArgument('Couldn\'t convert "{}" to PartialEmoji.'.format(argument))