Variable and parameter identifier should not depend on the type

[titorau]

Version
flatzinc 0.3.21

Use case
Get a parameter identifier.

Current behavior
I have to match the ParDeclItem over all types to get its id.

fn get_id(parameter: ParDeclItem) -> String {
    match parameter {
        ParDeclItem::Bool { id, bool: _ } => id,
        ParDeclItem::Int { id, int: _ } => id,
        ParDeclItem::Float { id, float: _ } => id,
        ParDeclItem::SetOfInt { id, set_literal: _ } => id,
        ParDeclItem::ArrayOfBool { ix: _, id, v: _ } => id,
        ParDeclItem::ArrayOfInt { ix: _, id, v: _ } => id,
        ParDeclItem::ArrayOfFloat { ix: _, id, v: _ } => id,
        ParDeclItem::ArrayOfSet { ix: _, id, v: _ } => id,
    }
}

Expected behavior
I can easily access the identifier via an attribute.

fn get_id(parameter: ParDeclItem) -> String {
    parameter.id
}

Remark
The problem is the same with for accessing a variable identifier (in VarDeclItem).