Properly support nested items or emit not-implemented error

[ironcev]

Currently, inside of the compiler we support nested items and only nested functions are explicitly not supported and having them in code will emit a not-implemented error. For other kind of items, the level of support varies and can end in different kinds of issues.

E.g., the below example compiles with errors shown:

script;

struct S { }

impl S {
    const C: u8 = 0; 
}

fn a() {
    struct S { }   // <<<--- ERROR: Name "S" is defined multiple times.
    
    impl S {
        const C: u8 = 1; // <<<--- ERROR: Duplicate definitions for the constant "C" for type "S".
    }
}

fn main() {
    a();
}

On the other hand, removing the outer S results in an ICE:

script;

fn a() {
    struct S { }
//  ^^^^^^^^^^^^ 
//  Internal compiler error: Unexpected struct declaration found.
//  Please file an issue on the repository and include the code that triggered this error.
    
    impl S {
        const C: u8 = 1;
    }
}

fn main() {
    a();
}