Tracking issue for organizing the standard library's #![feature]s

[m-ou-se]

The standard libraries use a lot of #![feature]s. We should organize them a bit and make sure it's acceptable for all of them to be used in the way they are.

It'd be nice to split them in language and library features. For the language features, we should check with the relevant team(s) if it's okay that we're using the feature in the library, and document with which of them we need to be extra careful and what their pitfalls are.

There's also a few features enabled that are no longer in use. We should remove those attributes.

• Categorize and clean up features
  • core: Core features cleanup #87768
  • alloc: Alloc features cleanup #87769
  • std
  • other libraries
• Make an overview of used language features
• Discuss those language features and their uses with the relevant team(s)
• Document stuff for future contributors and reviewers

[JmPotato]

Hi! I noticed that it had been a while since the last update of this issue, so I took some time to clean up the std library's ![feature]s. Could you please take a look at #94546, thx!