Add stable item representation

[xFrednet]

Create an initial version of the stable item representations.

Open Items:

• Module
• ExternCrate
• UseDeclaration
• Function
• TypeAlias
• Struct
• Enumeration
• Union
• ConstantItem
• StaticItem
• Trait
• Implementation
• ExternBlock

See: https://doc.rust-lang.org/reference/items.html

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This issue only focuses on the API representation. The driver backend will be added in a separate PR. This reduces the PR size and makes sure that I focus on the representation and not the way rustc stores the data :)

[xFrednet]

It might be cool to rework the UseDecl to not desugar into several use declarations like rustc does. This also depends on the reference.

[xFrednet]

I'll tick off the items I've already implemented in: https://github.com/xFrednet/rust-linting/tree/049-items-like-planets-in-space Wish me luck!