Auto merge of #76325 - lzutao:split-core-str, r=Amanieu

Split core/str/mod.rs to smaller files

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:
  ```bash
  git show --reverse --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space master..
  ```

---

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

* `converts`: Contains helper functions to convert from bytes to str.
* `error`: For error structs like Utf8Error.
* `iter`: For iterators of many str methods.
* `traits`: For indexing operations and build in traits on str.
* `validations`: For functions validating utf8 --- This name is awkward, maybe utf8.rs is better.

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