common element type for array and map literal

Allows more general styles of array and map literals.

```
class _Main {
	static function main(args : string[]) : void {
		var a = [1, null, 2]; // Array.<Nullable.<number>> i.e. Array.<number>
		var x = [1, null, "string"]; // Array.<variant>
		var y = {"a" : 1, "b" : null, "c" : "string"}; // Map.<variant>
	}
}
```