Clean up text handling a bit:

- Rename IO.write -> IO.print.
- Make IO.write not print a newline.
- Support \u Unicode escapes in strings.

test/variable/shadow_local.wren

@@ -2,7 +2,7 @@
   var a = "local"
   {
     var a = "shadow"
-    IO.write(a) // expect: shadow
+    IO.print(a) // expect: shadow
   }
-  IO.write(a) // expect: local
+  IO.print(a) // expect: local
 }