Remove the nightly feature and clean some parts of the code

[Kerollmops]

Now that the const_fn_trait feature is stable, we no longer need the nightly feature. I, therefore, remove the INSTRUMENTED_SYSTEM const in favor of the StatsAlloc::system const method. This is a breaking change. I removed one useless test related to this constant variable.

Questions

• What do you think of using the Ordering::Relaxed instead of the SeqCst. Operations will never be ordered anyway, so let's get more speed there.
• Can you publish a new release on crates.io?
• Would you mind if I could be co-owner of this crate and/or be able to publish versions, too?

[bIgBV]

I'm evaluating this crate and I'm also interested in using Ordering::Relaxed for operations. While adding this instrumentation is going to always add an overhead, I don't think we need the cost of sequential consistency in this usecase.