Clean up text handling a bit:

- Rename IO.write -> IO.print.
- Make IO.write not print a newline.
- Support \u Unicode escapes in strings.

benchmark/fib.wren

@@ -8,6 +8,6 @@ var fib = fn(n) {
 
 var start = OS.clock
 for (i in 1..5) {
-  IO.write(fib.call(28))
+  IO.print(fib.call(28))
 }
-IO.write("elapsed: " + (OS.clock - start).toString)
+IO.print("elapsed: " + (OS.clock - start).toString)