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/list/iterator_value.wren

@@ -1,5 +1,5 @@
 var a = ["one", "two", "three", "four"]
-IO.write(a.iteratorValue(0)) // expect: one
-IO.write(a.iteratorValue(1)) // expect: two
-IO.write(a.iteratorValue(2)) // expect: three
-IO.write(a.iteratorValue(3)) // expect: four
+IO.print(a.iteratorValue(0)) // expect: one
+IO.print(a.iteratorValue(1)) // expect: two
+IO.print(a.iteratorValue(2)) // expect: three
+IO.print(a.iteratorValue(3)) // expect: four