Incomplete features in `std`: required for implementation vs. should not use

[tgross35]

We currently allow incomplete_features in std but not core. It came up in #127739 (marking f16 and f128 as incomplete) that there is a difference between incomplete features that std needs to be able to work with or that are incomplete because of what is in std (such as the new types), and those that just should not be used.

Being able to allow specific incomplete features seems like it would be nice, if possible.

[tgross35]

@rustbot label +T-libs +T-compiler +A-lint -needs-triage

Cc @Nilstrieb @BoxyUwU per #127739 (comment)