Auto merge of #7875 - mbStavola:unit-hash, r=xFrednet

Add unit-hash lint

changelog: [`unit_hash`] Add lint for hashing unit values

This will lint for situations where the end user is attempting to hash a unit value (`()`), as the implementation in `std` simply [does nothing][impl]. Closes #7159 .

Example:

```rust
().hash(&mut state);

// Should (probably) be replaced with:
0_u8.hash(&mut state);
```

[impl]: https://github.com/rust-lang/rust/blob/a5f164faad4a2fed606b8160fd7ecd2d5cbba381/library/core/src/hash/mod.rs#L656

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