Rollup merge of #103701 - WaffleLapkin:__points-at-implementation__--this-can-be...

Simplify some pointer method implementations

- Make `pointer::with_metadata_of` const (+simplify implementation) (cc #75091)
- Simplify implementation of various pointer methods

r? ```@scottmcm```

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`from_raw_parts::<T>(this, metadata(self))` was annoying me for a while and I've finally figured out how it should _actually_ be done.

diff --cc library/core/src/lib.rs
Simple merge

diff --cc library/core/src/ptr/const_ptr.rs
Simple merge

diff --cc library/core/src/ptr/mut_ptr.rs
Simple merge