Refactor fuzzy help and clean up help command (#2122)

What's changed:
- Fixed issues mentioned on #2031
- Fuzzy help displays more like help manual
- Fuzzy help is easier and more flexible to use
- Fuzzy help string-matching ratio lowered to 80
- Help formatter is more extendable
- Help command has been optimized, cleaned up and better incorporates fuzzy help
- Added async_filter and async_enumerate utility functions because I was using them for this PR, then no longer needed them, but decided they might be useful anyway.
- Added `Context.me` property which is a shortcut to `Context.guild.me` or `Context.bot.user`, depending on the channel type.

redbot/cogs/alias/alias.py

@@ -1,6 +1,6 @@
 from copy import copy
 from re import search
-from typing import Generator, Tuple, Iterable
+from typing import Generator, Tuple, Iterable, Optional
 
 import discord
 from redbot.core import Config, commands, checks
@@ -53,10 +53,13 @@ class Alias:
         return (AliasEntry.from_json(d, bot=self.bot) for d in (await self._aliases.entries()))
 
     async def is_alias(
-        self, guild: discord.Guild, alias_name: str, server_aliases: Iterable[AliasEntry] = ()
-    ) -> (bool, AliasEntry):
+        self,
+        guild: Optional[discord.Guild],
+        alias_name: str,
+        server_aliases: Iterable[AliasEntry] = (),
+    ) -> Tuple[bool, Optional[AliasEntry]]:
 
-        if not server_aliases:
+        if not server_aliases and guild is not None:
             server_aliases = await self.unloaded_aliases(guild)
 
         global_aliases = await self.unloaded_global_aliases()