Auto merge of #49551 - scottmcm:deprecate-offset_to, r=KodrAus

Deprecate offset_to; switch core&alloc to using offset_from instead

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

cc https://github.com/rust-lang/rust/issues/41079

src/liballoc/lib.rs

@@ -99,11 +99,11 @@
 #![feature(lang_items)]
 #![feature(needs_allocator)]
 #![feature(nonzero)]
-#![feature(offset_to)]
 #![feature(optin_builtin_traits)]
 #![feature(pattern)]
 #![feature(pin)]
 #![feature(ptr_internals)]
+#![feature(ptr_offset_from)]
 #![feature(rustc_attrs)]
 #![feature(slice_get_slice)]
 #![feature(slice_rsplit)]