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/count.wren

@@ -1,3 +1,3 @@
-IO.write([].count)           // expect: 0
-IO.write([1].count)          // expect: 1
-IO.write([1, 2, 3, 4].count) // expect: 4
+IO.print([].count)           // expect: 0
+IO.print([1].count)          // expect: 1
+IO.print([1, 2, 3, 4].count) // expect: 4