[js] Optimize Std.string for basic types

This is a part of @deltaluca's JS optimization for Std.string, moved to api_inline as @ncannasse requested in #2255.

Code:
```haxe
class Main {
    static function main() {
        var x = 10;
        trace(Std.string(x));
        var x:UInt = 10;
        trace(Std.string(x));
        var x = 10.0;
        trace(Std.string(x));
        var x = "10";
        trace(Std.string(x));
        var x = true;
        trace(Std.string(x));
    }
}
```

JS Before:
```js
(function () { "use strict";
var Main = function() { };
Main.__name__ = true;
Main.main = function() {
	var x = 10;
	console.log(Std.string(x));
	var x1 = 10;
	console.log(Std.string(x1));
	var x2 = 10.0;
	console.log(Std.string(x2));
	var x3 = "10";
	console.log(Std.string(x3));
	var x4 = true;
	console.log(Std.string(x4));
};
var Std = function() { };
Std.__name__ = true;
Std.string = function(s) {
	return js.Boot.__string_rec(s,"");
};
var js = {};
js.Boot = function() { };
js.Boot.__name__ = true;
js.Boot.__string_rec = function(o,s) {
	if(o == null) return "null";
	if(s.length >= 5) return "<...>";
	var t = typeof(o);
	if(t == "function" && (o.__name__ || o.__ename__)) t = "object";
	switch(t) {
	case "object":
		if(o instanceof Array) {
			if(o.__enum__) {
				if(o.length == 2) return o[0];
				var str = o[0] + "(";
				s += "\t";
				var _g1 = 2;
				var _g = o.length;
				while(_g1 < _g) {
					var i = _g1++;
					if(i != 2) str += "," + js.Boot.__string_rec(o[i],s); else str += js.Boot.__string_rec(o[i],s);
				}
				return str + ")";
			}
			var l = o.length;
			var i;
			var str = "[";
			s += "\t";
			var _g = 0;
			while(_g < l) {
				var i1 = _g++;
				str += (i1 > 0?",":"") + js.Boot.__string_rec(o[i1],s);
			}
			str += "]";
			return str;
		}
		var tostr;
		try {
			tostr = o.toString;
		} catch( e ) {
			return "???";
		}
		if(tostr != null && tostr != Object.toString) {
			var s2 = o.toString();
			if(s2 != "[object Object]") return s2;
		}
		var k = null;
		var str = "{\n";
		s += "\t";
		var hasp = o.hasOwnProperty != null;
		for( var k in o ) {
		if(hasp && !o.hasOwnProperty(k)) {
			continue;
		}
		if(k == "prototype" || k == "__class__" || k == "__super__" || k == "__interfaces__" || k == "__properties__") {
			continue;
		}
		if(str.length != 2) str += ", \n";
		str += s + k + " : " + js.Boot.__string_rec(o[k],s);
		}
		s = s.substring(1);
		str += "\n" + s + "}";
		return str;
	case "function":
		return "<function>";
	case "string":
		return o;
	default:
		return String(o);
	}
};
String.__name__ = true;
Array.__name__ = true;
Main.main();
})();
```

JS After:
```js
(function () { "use strict";
var Main = function() { };
Main.main = function() {
	var x = 10;
	console.log("" + x);
	var x1 = 10;
	console.log("" + x1);
	var x2 = 10.0;
	console.log("" + x2);
	var x3 = "10";
	console.log(x3);
	var x4 = true;
	console.log("" + x4);
};
Main.main();
})();
```