Void->Void unification not strict enough

```haxe
class Main {
public static function main() {
foo(() -> function() {
trace('whoops'); // Never called, anyway
});
}

static function foo(cb:Void->Void) {
cb();
}
}
```

I was lacking coffee and made this "typo" while refactoring some code... and haxe doesn't complain (both 4.0.5 and recent dev (`af197e3` from 2020-04-09)).

Start by compiling the provided Main example with Haxe 4.0.5 or the referenced development revision and confirm that the callback is accepted. Trace the compiler's function-type unification or type-checking path for Void->Void; done means this example is rejected with a useful type error while valid Void->Void callbacks still compile.

Written by the indexing model from the issue text.