Consider following code:

    immutable struct Foo {
        this(int) { }
        
        void bar() { }
    }
    
    int main(in string[] args) {
        auto foo = Foo(1);
        
        foo.bar();
    }

This will not work out, because `bar()` is immutable, but `foo` is not. If constructor is declared as `this(int) immutable {}`, it will work again. Default no-arg constructor for immutable structs is already immutable, so shouldn't other constructor also be considered immutable implicitly, much like methods are?