Feature Request: Define command REPLs on command types

[Hirrolot]

Currently, crate::{commands_repl, commands_repl_with_listener} accept a command type hint as a third parameter. I pondered awhile on how not to require a type hint and came to a stupid solution: just define REPL methods on command types themselves. I.e., we could introduce one more trait with the commands_repl(_with_listener) methods and define it for Cmd: BotCommands, we don't even need to change the macro library.

- teloxide::commands_repl(bot, answer, Command::ty())
+ Command::repl(bot, answer)

Pros

• More concise syntax -- no need to specify that weird type hint.

Cons

• Need to deprecate the old command REPLs.
• Maybe trait methods are more tricky to discover in the API?

Alternatives

• Leave the design as-is.

[WaffleLapkin]

This proposed API makes a lot more sense than Command::ty, I think we can sacrifice a little discoverability. LGTM.

[Hirrolot]

Implemented in #746.