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/setter/instance.wren

@@ -1,6 +1,6 @@
 class Foo {
   bar = value {
-    IO.write(value)
+    IO.print(value)
   }
 }