Better inlining of for-loops with anonymous iterators.

[RealyUniqueName]

This PR allows to inline for loops with Iterator or Iterable provided as type parameter without allocating an iterator.
Here is an example:

Main.hx

class Main {
  public static function main() {
      iter(new Collection(10));
  }

  static public inline function iter<A,T:Iterable<A>>(v:T) {
      for(i in v) {
          trace(i);
      }
  }
}

class Collection<V> {
  public var amount:Int;
  public inline function new(amount:Int) this.amount = amount;
  public inline function iterator() return new CollectionIterator(this);
}

class CollectionIterator<V> {
  var collection:Collection<V>;
  var current:Int = 0;
  public inline function new(collection:Collection<V>) this.collection = collection;
  public inline function hasNext() return current++ < collection.amount;
  public inline function next() return (null:V);

}

JS compiled _with_ this PR

// Generated by Haxe 4.0.0-rc.5
(function ($global) { "use strict";
var Main = function() { };
Main.main = function() {
    var _g_collection_amount = 10;
    var _g_current = 0;
    while(_g_current++ < _g_collection_amount) {
        var i = null;
        console.log("src/Main.hx:8:",i);
    }
};
Main.main();
})({});

JS compiled _without_ this PR

// Generated by Haxe 4.0.0-rc.5
(function ($global) { "use strict";
var Main = function() { };
Main.main = function() {
    var i = new CollectionIterator(new Collection(10));
    while(i.hasNext()) {
        var i1 = i.next();
        console.log("src/Main.hx:8:",i1);
    }
};
var Collection = function(amount) {
    this.amount = amount;
};
Collection.prototype = {
    iterator: function() {
        return new CollectionIterator(this);
    }
};
var CollectionIterator = function(collection) {
    this.current = 0;
    this.collection = collection;
};
CollectionIterator.prototype = {
    hasNext: function() {
        return this.current++ < this.collection.amount;
    }
    ,next: function() {
        return null;
    }
};
Main.main();
})({});

If this PR is accepted I'll make another one which will make Lambda methods allocation-free