Support *SCAN methods on async connections

[swilcox3]

Following the pattern set by Iter, I implemented an AsyncIter and exposed it up through AsyncCommands. Thanks to @Marwes who helped me a lot.
Unfortunately I'm on Windows, so I don't have redis-server installed and I can't run the tests myself. I usually use Docker to run redis.

[swilcox3]

Oh, this doesn't work because the implementations of aio::ConnectionLike are calling get_packed_command, not get_packed_command_with_cursor. I'll fix that.

[swilcox3]

OK fixed. It's a breaking change though because I had to change the signature of aio::ConnectionLike to match the signature of ConnectionLike, replacing the Cmd with &[u8].

[swilcox3]

Finally, got the tests to pass. Man, it's crippling when you can't run them all. The only failure now is clippy saying that we should name the next() method something else because it resembles Iterator. I like that it resembles the Stream syntax, but it could definitely be confusing. Should we rename the function or allow the warning?

[Marwes]

I'd rename it to next_item or something then.

I guess you could get tests working locally if you tweak the tests to read an environment variable which contains a custom command to run instead of redis and use that start redis in a docker instance,

[Terkwood]

Is this duplicate value somehow necessary? Curious as I'm working on a merge into #319. I think this can be trimmed.

Thanks!

[Marwes]

Yeah, that can probably be removed.

[Terkwood]

Cool, I've updated #319. Cheers.