Handle all entity types on isinstance checks

Only the uses of `isinstance` against `InputPeer*` types were
reviewed. Notably, `utils` is exempt on this because it needs
to deal with everything on a case-by-case basis.

Since the addition of `*FromMessage` peers, any manual `isinstance`
checks to determine the type were prone to breaking or being
forgotten to be updated, so a common `helpers._entity_type()`
method was made to share this logic.

Since the conversion to `Peer` would be too expensive, a simpler
check against the name is made, which should be fast and cheap.

telethon/client/downloads.py

@@ -257,7 +257,8 @@ class DownloadMethods:
             # See issue #500, Android app fails as of v4.6.0 (1155).
             # The fix seems to be using the full channel chat photo.
             ie = await self.get_input_entity(entity)
-            if isinstance(ie, types.InputPeerChannel):
+            ty = helpers._entity_type(ie)
+            if ty == helpers._EntityType.CHANNEL:
                 full = await self(functions.channels.GetFullChannelRequest(ie))
                 return await self._download_photo(
                     full.full_chat.chat_photo, file,