Improve file/directory utils (#84)

test/io/directory/create_delete.wren

@@ -0,0 +1,23 @@
+import "io" for Directory, Stat
+import "os" for Platform
+
+if (Directory.exists("tmp")) {
+    Directory.delete("tmp")
+}
+
+System.print(Directory.exists("tmp")) // expect: false
+Directory.create("tmp")
+System.print(Directory.exists("tmp")) // expect: true
+
+// Windows does not support mode
+if (Platform.isPosix) {
+    var stat = Stat.path("tmp")
+    // 511 is 0755
+    System.print(stat.mode && 0x1ff) // expect: 511
+}
+
+Directory.delete("tmp")
+System.print(Directory.exists("tmp")) // expect: false
+
+
+