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/while/closure_in_body.wren

@@ -3,7 +3,7 @@ var list = []
 var i = 1
 while (i < 4) {
   var j = i + 1
-  list.add(fn IO.write(j))
+  list.add(fn IO.print(j))
   i = i + 1
 }