Add links in docs for some primitive types
This pull request adds additional links in existing documentation of some of the primitive types.
Where items are linked only once, I have used the `[link](destination)` format. For items in `std`, I have linked directly to the HTML, since although the primitives are in `core`, they are not displayed on `core` documentation. I was unsure of what length I should keep lines of documentation to, so I tried to keep them within reason.
Additionally, I have avoided excessively linking to keywords like `self` when they are not relevant to the documentation. I can add these links if it would be an improvement.
I hope this can improve Rust. Please let me know if there's anything I did wrong!
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