Allow std to re-export unstable things from core without having to enable the feature itself

[m-ou-se]

library/std/src/lib.rs now enables a lot of #![feature]s from core and alloc and test only because it needs to re-export some unstable items (with the same #[unstable] tag). It makes it hard to tell if std needs those features itself or not.

It'd be great if an #[unstable] pub use would work without enabling the feature in the entire crate. (As long as the feature name is the same as on the original item, I suppose.)

These are all not directly used by std, but only used for items that are unstably re-exported:

#![feature(assert_matches)]
#![feature(async_iterator)]
#![feature(c_variadic)]
#![feature(cfg_accessible)]
#![feature(cfg_eval)]
#![feature(concat_bytes)]
#![feature(const_format_args)]
#![feature(c_size_t)]
#![feature(core_ffi_c)]
#![feature(core_panic)]
#![feature(custom_test_frameworks)]
#![feature(edition_panic)]
#![feature(format_args_nl)]
#![feature(log_syntax)]
#![feature(once_cell)]
#![feature(saturating_int_impl)]
#![feature(stdsimd)]
#![feature(test)]
#![feature(trace_macros)]

I'd love to remove these lines.

[semicoleon]

@rustbot claim

[semicoleon]

Do we want a separate issue for actually removing those lines once the change reaches beta, or should we reopen this one?