Discord-ext-slash

Fixes
* `Context.send` now returns the message like it should
* `SlashBot` now properly handles explicitly passing `None` for `debug_guild`
* The `Command` constructor now raises `ValueError` if no description can be found or inferred

Fixes
* Forthport changes from [0.2.2][0.2.2]
* More annotations
* Clean up

Fixes
* `in_addition` is removed, so remove it from the demo
* `slash.__all__` is now defined
* Support forward reference annotations
* The message of the `ValueError` raised when no argument is annotated with `Context` was written when that was still named `SlashContext`

New Stuff
* `Group.default` on top of `Group.slash_cmd()` marks a subcommand as the one called when the base group is invoked.

Changes
* `default` behavior is [fully clarified](https://github.com/discord/discord-api-docs/issues/2393#issuecomment-767733336) now, but not currently enabled.
* This means `Option.default` is fully removed now.
* Since calling a base group translates into calling the `default` subcommand, the `Group.coro` is now deprived of an original jurisdiction. Final behavior is that all parent coros will be called in order of increasing child levels before the command is invoked.

Fixes:
* Actual Python annotations for classes, instead of just doc descriptions

New Stuff
* For some extra security, fall back to looking up command by name and guild ID if command ID fails to return results. Fall back from that to just name. Warn with `SlashWarning` both times. Error on still no command.

Fixes
* The API introduced a new "version" dict key, which broke dict equality. I gave up on that and just ensured all values in code match the API.
* The API no longer allows passing `name` in PATCH. Pop it.