Remove func.dart and use Dart 2's Function syntax

[ditman]

func.dart has this comment:

TODO: once Dart v1 support is dropped, use new Function syntax instead

Looking at the pubspec, it seems Dart 1 was dropped long ago (min SDK version is 2.7.0), but that file, and some usages of the Func types defined there still persist.

(It may have also caused issues like this.)

The file should be removed, and the FuncX types rewritten as modern Function types.