Use dep: syntax for portable-atomic

[jamesmunns]

Fixes #170

[Dirbaio]

Why not name the feature portable-atomic? It's confusing that the feature is named portable_atomic but the crate is named portable-atomic.

Also this could be fixed without a breaking change, changing the code from cfg(feature="portable_atomic") to cfg(feature="portable-atomic"). This way it works both if the user enables portable-atomic or portable_atomic. The portable_atomic name can be marked as deprecated.

[jamesmunns]

I don't have any opinion here. Happy to have my PR superceded if you'd prefer to open an alternative @Dirbaio