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/variable/local_in_nested_block.wren

@@ -1,6 +1,6 @@
 {
   var a = "outer"
   {
-    IO.write(a) // expect: outer
+    IO.print(a) // expect: outer
   }
 }