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/use_global_in_function.wren

@@ -2,5 +2,5 @@ var global = "global"
 // TODO: Forward reference to global declared after use.
 
 fn {
-  IO.write(global) // expect: global
+  IO.print(global) // expect: global
 }.call