Add reexport of slice::from{,_mut}_ptr_range to alloc & std

At first I was confused why `std::slice::from_ptr_range` didn't work :D

Author: WaffleLapkin

library/alloc/src/lib.rs

@@ -130,6 +130,7 @@
 #![feature(ptr_sub_ptr)]
 #![feature(receiver_trait)]
 #![feature(set_ptr_value)]
+#![feature(slice_from_ptr_range)]
 #![feature(slice_group_by)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]

library/alloc/src/slice.rs

@@ -114,6 +114,8 @@ pub use core::slice::EscapeAscii;
 pub use core::slice::SliceIndex;
 #[stable(feature = "from_ref", since = "1.28.0")]
 pub use core::slice::{from_mut, from_ref};
+#[unstable(feature = "slice_from_ptr_range", issue = "89792")]
+pub use core::slice::{from_mut_ptr_range, from_ptr_range};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::slice::{from_raw_parts, from_raw_parts_mut};
 #[stable(feature = "rust1", since = "1.0.0")]