[lua] small std tweaks

std/lua/_std/Array.hx

@@ -48,8 +48,7 @@ class Array<T> {
 		return ret;
 	}
 	public function push(x : T) : Int {
-		lua.Lua.rawset(untyped this, length,x);
-		length++;
+		this[this.length] = x;
 		return length;
 	}
 	public function reverse() : Void {

std/lua/_std/String.hx

@@ -89,7 +89,7 @@ class String {
 				ret.push(match);
 				idx = newidx + delimiter.length;
 			} else {
-				ret.push(NativeStringTools.sub(this,idx,NativeStringTools.len(this)).match);
+				ret.push(NativeStringTools.sub(this,idx,this.length).match);
 				idx = null;
 			}
 		}