Discord-ext-slash

Major Fixes
* Because Group now inherits from Command, and all Groups were being instance-checked as Command first, subcommands ended up never being called... Switched the order.
* The stock check couldn't accept the self argument, so cog commands all failed.

New Stuff
* `slash.Context` subclasses can now be used to annotate the context argument. They will be instantiated with the same signature as the base class.

Changes
* Disabled `default` argument to `Option`. It will now always be `False`. [The argument is not supported yet.](https://github.com/discord/discord-api-docs/issues/2393#issuecomment-765800187) The library will receive another update when it becomes supported.

New Stuff
* Base command groups will be able to be invoked directly at some point in the future, so that is supported now
* This means that the decorated coro is actually the callback
* Checks have been moved into `Group.check`
* You can specify `in_addition=True` to call the group callback before a subcommand callback
* `MessageFlags` enum marks message flags
* In particular, `MessageFlags.EPHEMERAL` can be passed into the new `flags` argument in `Context.respond()` to send a message à la Clyde that is private and dismissable

Changes
* The entire traceback for failed fetches when constructing `Context`s is no longer logged, only the fact that the fetch failed.
* Some objects now have `__repr__`s or `__str__`s
* `Group` inherits from `Command`

Fixes
* Logger name was wrong (correct is `discord.ext.slash`)
* `allowed_mentions` was actually supported by the API this whole time, but my implementation was faulty

Changes
* The `cog_check` method of a cog is now supported (though **only as an async method**)

Fixes
* `SlashBot.debug_guild` wasn't being casted to `int`, and it was being set before calling `super()`
* `content` is now blindly casted to `str`

Changes
* Cogs are actually a thing now, more or less. `Command.cog`, `Group.cog`, `Context.cog` have values now.
<br/>This was done because instance methods weren't being called with a `self` argument
* Groups copy their cog to their subcommands
* `Context.respond()` now takes `embed` as shorthand for one-item `embeds`

Fixes
* Options with names other than the function argument name used to break.
* Subcommands with no arguments used to not be callable.
* Critical: Not specifying a guild ID led to failure to register the command
* Subcommands in cogs were being doubly added as top-level commands
* If one command is invalid, this doesn't prevent the rest from being registered

A few bugfixes

* `id`s were not `int`s in a lot of places.
* Processing `Context.options` did not have error handling.
* It turns out that `Choice.value` cannot be `int`.
* Changed doc of `Command` to say "parameters" because `check` is not an attribute