Support doc comments and attributes on nested items

[ironcev]

E.g., while this compiles:

library;

fn fun() {
    struct S {
        field: u8,
    }

    const CONST: u8 = 0;
}

trying to attribute or comment nested items will result in "Expected an expression." error on every doc comment or attribute:

library;

fn fun() {
    /// Comment.             // <<<--- ERROR: Expected an expression.
    #[allow(dead_code)]      // <<<--- ERROR: Expected an expression.
    struct S {
        /// Comment.         // <<<--- ERROR: Expected an expression.
        #[allow(dead_code)]  // <<<--- ERROR: Expected an expression.
        field: u8,
    }

    /// Comment.             // <<<--- ERROR: Expected an expression.
    #[allow(dead_code)]      // <<<--- ERROR: Expected an expression.
    const CONST: u8 = 0;
}