c: add support for structs

[YerinAlexey]

Description

This patch adds support for structs in C generator.

ToDo

• [x] Generate struct definition
• [x] Generate struct type (type annotations, nested structs)
• [x] Struct creation (new User { ... })
• [x] Field access

[YerinAlexey]

Clippy test fails but this doesn't seem related to this PR

[garritfra]

@YerinAlexey it is. format!() needs a .into() in line 290.

[YerinAlexey]

@YerinAlexey it is. format!() needs a .into() in line 290.

No, format!() already returns a String

[garritfra]

Are you sure? I thought it's &str, which is not owned.

[YerinAlexey]

https://doc.rust-lang.org/std/fmt/fn.format.html

[YerinAlexey]

I fixed clippy issues, they were caused because of a wrong return and using format!("{{"} instead of String::from("{")