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/setter/result.wren

@@ -3,4 +3,4 @@ class Foo {
 }
 
 var foo = new Foo
-IO.write(foo.bar = "value") // expect: result
+IO.print(foo.bar = "value") // expect: result