Allow overriding the default generation of rest arguments in js-es <= 5

[R32]

This gives the option to use the code generated by the compiler or not.

Compared to copying elements to a new array,
I prefer a cleaner way, Even if the V8 VM would complain: 'Bad value context for arguments value.'

function rest( name : String, args : haxe.Rest<String> ) {
#if (js_es < 6)
    var args : Array<String> = js.Syntax.code("Array.prototype.slice.call(arguments, 1)"); // overrides args
#end
    for (s in args)
        trace(name + " : " + s);
}

generated :

function rest(name) {
	var args = Array.prototype.slice.call(arguments, 1);
	var _g = 0;
	while(_g < args.length) console.log("src/Main.hx:27:",name + " : " + args[_g++]);
}