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/shadow_global.wren

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