The stable feature seems backwards

[Lokathor]

With the current setup, the stable feature allows you to build the crate on stable, and removing it adds some speed boosts but requires nightly. However, once a dependency is flipped no by any crate in the graph, all crates get that feature on, so features should be purely additive whenever possible.

Accordingly, the meaning should be flipped around: stable is default, and unstable intrinsic usage is enabled with a feature flag being turned on if the user wants that boost.

[Lokathor]

Closed by #224