Add some feature flags we'll need soonish
#![feature(doc_cfg)]
#![feature(core_intrinsics)]
#![feature(const_fn)]
+#![feature(const_if_match)]
+#![feature(const_raw_ptr_to_usize_cast)]
#![feature(const_generics)]
#![feature(maybe_uninit_ref)]
#![feature(maybe_uninit_extra)]
#![feature(trusted_len)]
#![feature(slice_partition_dedup)]
#![feature(read_initializer)]
+#![feature(specialization)]
//Literally just for stable-sort.
#[cfg(any(feature = "std", rustdoc))]
