stabilize valuable in 0.1.x

[mladedav]

Motivation

valuable support has had unstable support for two years now with no major issues. The crate has close to 30 million downloads on crates.io and all its dependants with more than 3 million downloads are tracing in this repository so it is safe to say that some people have used it.

There seem to be no blockers so I think that this could be stabilized and given to anyone who has not opted-in into the unstable features.

Solution

The unstable feature checks were removed as well as its mentions. Cargo feature is still present.

Potential Issues

• Turning the feature on only for tracing but not for tracing-subscriber. There seem to have been instances (here and here) where people did this and then were surprised by the (json) subscriber using fmt::Debug instead of a specialized implementation.
  This can be solved by removing the cargo feature completely without providing an option to opt-out of valuable usage.

  This also adds a ~ sigil to macros for valuable support. These two changes can be easily split if we put the macro and test changes after the stabilization, but there would be a lot of new feature gaurds (which would hopefully be short-lived).