pin to async-task 1.2.1

[yoshuawuyts]

We should pin async-task to 1.2.1 to include async-rs/async-task#15 and async-rs/async-task#16, together adding no_std support. We should also document how to enable no_std support in async_std.

cc/ @skade @jamesmunns

[k-nasa]

@yoshuawuyts
Could you tell me the advantages of no_std ❓
I'm not negative about this, but I don't understand what would be better.
However, I am interested in working on this.

[yoshuawuyts]

@k-nasa no_std is for use in embedded systems, making it possible to run code on microcontrollers and the like.

After talking with folks a bit offline, it seems we'll need to do more work to enable no_std in async_std. But upgrading to the new async-task API would be a great first step.

[k-nasa]

@yoshuawuyts Thanks!!
I understand roughly.
Can net module and fs be migrated to no_std?
I don't know.

[yoshuawuyts]

Possibly in the future; it'd be a lot of work though and I think it's quite experimental still.

The shorter term goal would be to get parts of async-std working on no_std. It's am open question of which parts, and how we would approach that.

[k-nasa]

I modified some modules (future, task, stream) to work with no_std.
I don't know if this is appropriate, but I hope it helps.

ref: #680