Put back std #![feature]s that were not unused.

Author: m-ou-se

library/std/src/lib.rs

@@ -279,6 +279,7 @@
 #![feature(portable_simd)]
 #![feature(ptr_as_uninit)]
 #![feature(raw_os_nonzero)]
+#![feature(slice_internals)]
 #![feature(slice_ptr_get)]
 #![feature(std_internals)]
 #![feature(str_internals)]
@@ -327,6 +328,7 @@
 #![feature(const_ip)]
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
+#![feature(const_option)]
 #![feature(const_socketaddr)]
 #![feature(thread_local_internals)]
 //