Rollup merge of #100822 - WaffleLapkin:no_offset_question_mark, r=scottmcm

Replace most uses of `pointer::offset` with `add` and `sub`

As PR title says, it replaces `pointer::offset` in compiler and standard library with `pointer::add` and `pointer::sub`. This generally makes code cleaner, easier to grasp and removes (or, well, hides) integer casts.

This is generally trivially correct, `.offset(-constant)` is just `.sub(constant)`, `.offset(usized as isize)` is just `.add(usized)`, etc. However in some cases we need to be careful with signs of things.

r? ````@scottmcm````

_split off from #100746_

compiler/rustc_serialize/src/serialize.rs

@@ -273,7 +273,7 @@ impl<D: Decoder, T: Decodable<D>> Decodable<D> for Vec<T> {
         unsafe {
             let ptr: *mut T = vec.as_mut_ptr();
             for i in 0..len {
-                std::ptr::write(ptr.offset(i as isize), Decodable::decode(d));
+                std::ptr::write(ptr.add(i), Decodable::decode(d));
             }
             vec.set_len(len);
         }