Deprecate offset_to; switch core&alloc to using offset_from instead

Bonus: might make code than uses `.len()` on slice iterators faster

src/liballoc/lib.rs

@@ -103,13 +103,13 @@
 #![feature(lang_items)]
 #![feature(needs_allocator)]
 #![feature(nonzero)]
-#![feature(offset_to)]
 #![feature(optin_builtin_traits)]
 #![feature(pattern)]
 #![feature(pin)]
 #![feature(placement_in_syntax)]
 #![feature(placement_new_protocol)]
 #![feature(ptr_internals)]
+#![feature(ptr_offset_from)]
 #![feature(rustc_attrs)]
 #![feature(slice_get_slice)]
 #![feature(slice_rsplit)]