Add features for queues.

[reitermarkus]

Partially addresses #372.

[Dirbaio]

Have you measured how much compile time this saves?

IMO features make sense if they prevent pulling in big dependencies, but in this case they just prevent compiling a few hundred lines of code. In this case the compile time difference is probably negligible, and not worth the dev-experience downgrade due to users having to enable features.

[reitermarkus]

No, I didn't measure compile time.

Should we just close #372 then? The only non-optional dependency we have is hash32, which is pretty tiny as well, with no dependencies itself.