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/this/this_in_method.wren

@@ -3,4 +3,4 @@ class Foo {
   baz { return "baz" }
 }
 
-IO.write((new Foo).bar.baz) // expect: baz
+IO.print((new Foo).bar.baz) // expect: baz