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/for/only_evaluate_sequence_once.wren

@@ -1,9 +1,9 @@
 var f = fn {
-  IO.write("evaluate sequence")
+  IO.print("evaluate sequence")
   return [1, 2, 3]
 }
 
-for (i in f.call) IO.write(i)
+for (i in f.call) IO.print(i)
 // expect: evaluate sequence
 // expect: 1
 // expect: 2