Resolver should check for unused variables

[Gusarich]

For example, in the case below only the y variable is actually used and we don't need x at all.

house TestHouse {
    global counter: int = 0;

    handle(event: JoinEvent) {
        let x: int = 1;
        let y: int = 2;
        global.counter = global.counter + y;
    }
}

Same should apply for unused function arguments once #7 is resolved

[Gusarich]

I think it'll be better to make it as Compilation Warning instead of a fatal error, so let's move it to the 1.0.0 milestone because it's not urgent at all