Introduce immutable modifier

[biqas]

Introducing immutable modifier on class, field, property, event, indexer and method level.

Benefits could be:

• could help express the intention to better understand the class design and behavior
• could make some assumption/prediction/facts for garbage collection.
• could have some inference chain for not mixing immutable with mutable things.
• could have positive effects for lock's

public immutable class A
{
    // so everything is immutable in the class
    public A Foo()
    {
        // not allowed.
        // return this;

        return new A();
    }
}

public class A
{
    private immutable int fieldA = 3;

    public A()
    {
        // allowed
        this.fieldA = 0;
    }

    public immutable A Foo()
    {
        // not allowed !!!
        this.fieldA = 42;

        return new A();
    }
}

So chain of immutable must be inspected to guarantee immutable remains immutable.

[pharring]

@biqas Thanks for the proposal. Immutability and immutable types are discussed in #159. Please review it and see if it covers all the scenarios you were thinking of. If there's something else, it's best to discuss it there.

Note that readonly on fields already takes care of your second example.