Auto merge of #76327 - lzutao:split-core-num, r=SimonSapin

Split `core/num/mod.rs` to smaller mods

Note for reviewer:
* I split to multiple commits for easier reviewing, but I could git squash them all to one if requested.
* Recommend pulling this change locally and using advanced git diff viewer or this command:
  ```
  git show --reverse --color-moved=dimmed-zebra master..
  ```

---

I split `core/num/mod.rs` to these modules:

* `error`: For error structs like `ParseIntError`.
* blanket `shells` dir: For dummy number type modules: std::i32, std::f32, and the likes. Why? See below.
* `int_macros` and `uint_macros`: Real implementation of all integer types via `int_impl` and `uint_impl`
* `nonzero`: For `NonZero*` types and their implementations.
* `wrapping`: For `Wrapping<T>` types.

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