Allocation free Map.iterator() and Map.keys()

[RealyUniqueName]

Ok, not completely free on all targets (e.g. we still need an array for keys on js), but close to that :)

Closes #3147

This branch contains only JS implementation for Map.iterator and Map.key to illustrate the idea.
The idea is to move iterator and key to @:using.
The same approach could be applied to Map.keyValueIterator().

class Main {
	static function main() {
		var m = [1 => 2, 3 => 4];
		for(i in m) {
			trace(i);
		}
	}
}

Generated JS with current development

// Generated by Haxe 4.0.0-rc.5
(function ($global) { "use strict";
var HxOverrides = function() { };
HxOverrides.iter = function(a) {
	return { cur : 0, arr : a, hasNext : function() {
		return this.cur < this.arr.length;
	}, next : function() {
		return this.arr[this.cur++];
	}};
};
var Main = function() { };
Main.main = function() {
	var _g = new haxe_ds_IntMap();
	_g.h[1] = 2;
	_g.h[3] = 4;
	var i = _g.iterator();
	while(i.hasNext()) console.log("src/Main.hx:5:",i.next());
};
var haxe_ds_IntMap = function() {
	this.h = { };
};
haxe_ds_IntMap.prototype = {
	keys: function() {
		var a = [];
		for( var key in this.h ) this.h.hasOwnProperty(key) ? a.push(key | 0) : null;
		return HxOverrides.iter(a);
	}
	,iterator: function() {
		return { ref : this.h, it : this.keys(), hasNext : function() {
			return this.it.hasNext();
		}, next : function() {
			var i = this.it.next();
			return this.ref[i];
		}};
	}
};
Main.main();
})({});

Generated JS with this PR

// Generated by Haxe 4.0.0-rc.5
(function ($global) { "use strict";
var Main = function() { };
Main.main = function() {
	var _g1_map_h = { };
	_g1_map_h[1] = 2;
	_g1_map_h[3] = 4;
	var a = [];
	for( var key in _g1_map_h ) _g1_map_h.hasOwnProperty(key) ? a.push(key | 0) : null;
	var keys = a;
	var _g1_keys = keys;
	var _g1_index = 0;
	var _g1_count = keys.length;
	while(_g1_index < _g1_count) console.log("src/Main.hx:5:",_g1_map_h[_g1_keys[_g1_index++]]);
};
Main.main();
})({});

[Gama11]

Relevant issue: #3147

[RealyUniqueName]

For clarity:
With current development that sample allocates an array for keys, an iterator for keys, an iterator for values.
With this PR that sample only allocates an array for keys.

[nadako]

Is this a (temporary?) workaround for #7379? I don't like that we have to go the weird way for such a basic thing...

[RealyUniqueName]

I think for iterating @:multiType we don't have an alternative currently. The best we can do is replace @:multiType entirely with a better feature.

[Simn]

I don't have a strong opinion on this one, so I'll let the two of you fight this out.

[RealyUniqueName]

I'd like to finish and merge this PR, because I doubt we will get an alternative for @:multiType any time soon.

[nadako]

I don't like this and I'm not sure why I'm assigned to it. As far as I understand something like #9432 will fix the original issue, so IMO it's better to go that way.

[Simn]

Looking at this again and considering all the possible problems, it's better to not do this. #7379 is still open to track the original problem.