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/closure/reference_closure_multiple_times.wren

@@ -3,8 +3,8 @@ var f = null
 {
   var a = "a"
   f = fn {
-    IO.write(a)
-    IO.write(a)
+    IO.print(a)
+    IO.print(a)
   }
 }