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/many_locals.wren

@@ -255,5 +255,5 @@
   var a253 = a252
   var a254 = a253
   var a255 = a254
-  IO.write(a255) // expect: value
+  IO.print(a255) // expect: value
 }