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/iterate.wren

@@ -1,7 +1,7 @@
 var a = ["one", "two", "three", "four"]
-IO.write(a.iterate(null)) // expect: 0
-IO.write(a.iterate(0)) // expect: 1
-IO.write(a.iterate(1)) // expect: 2
-IO.write(a.iterate(2)) // expect: 3
-IO.write(a.iterate(3)) // expect: false
-IO.write(a.iterate(-1)) // expect: false
+IO.print(a.iterate(null)) // expect: 0
+IO.print(a.iterate(0)) // expect: 1
+IO.print(a.iterate(1)) // expect: 2
+IO.print(a.iterate(2)) // expect: 3
+IO.print(a.iterate(3)) // expect: false
+IO.print(a.iterate(-1)) // expect: false