feat: new channels

[dignifiedquire]

• add new top level channel module
• reimplement existing channels using async-channel

[ghost]

Looks like channel module is in the wrong place: it's in async_std::channel, while it should be async_std::sync::channel

[yoshuawuyts]

Looks like channel module is in the wrong place: it's in async_std::channel

@stjepang that's the intent; the libs team has been talking about possibly breaking std::sync up into smaller bits anyway, and we figured a channel impl would be a good candidate for this. But now that you know it's intentional I'm curious to hear what you think!

[ghost]

Ha, interesting! I actually like channel as a top-level module and wish std did the same.

[dignifiedquire]

Going to close this for now. Will rework later.