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/bool/to_string.wren

@@ -1,2 +1,2 @@
-IO.write(true.toString)  // expect: true
-IO.write(false.toString) // expect: false
+IO.print(true.toString)  // expect: true
+IO.print(false.toString) // expect: false