Create default `stable` feature before 1.0 to avoid future breaking changes

[paulocsanz]

Hey, this comes from a discussion that happened on reddit after a announcement of this crate, basically it would be nice to split this crate in features in the future, to allow using just a fraction of it. This would make it incredibly easy to integrate this crate with others in the async ecosystem.

But splitting into features is a breaking change, since it will break the crate for whoever uses default-features = false, so either we do it before 1.0, or we will have to wait for 2.0.

There is a trick tho, we can avoid this breaking change if we crate a stable feature right now and put everything stable in it, so default-features = false wouldn't work and they would be forced to select the features from the start.

Discussion:
https://www.reddit.com/r/rust/comments/doos98/asyncstd_v09911_released/f5phwor/

[skade]

I would generally consider this. A good question though is if we want this attribute in a broad fashion, like your PR or in a granular fashion like the in the stdlib (annotating every item!).

The latter gives a lot of granularity, but gives a lot of churn now. Later, it is easier for people to apply function by function.

Also, I was suggesting something similar for WASM here: #225 (comment)

Aside: I would really like if we had the stable attribute outside of stdlib...