Forbid @NonNull on java.lang.Void

[wmdietl]

@Nullable is the default for Void.

However, nothing forbids someone from writing:

import org.checkerframework.checker.nullness.qual.NonNull;
class VoidUse {
  void foo(@NonNull Void v) {
      v.toString();
  }
}

This passes the Nullness Checker without a warning.
The only possible call of that method would pass null, the only value of type Void.

It's unlikely that anyone would write this code, but we should forbid explicit usage of @NonNull on Void.

We have an UpperBound annotation, but not a corresponding LowerBound annotation.
Void should have a lower bound of @Nullable to forbid uses with @NonNull.

It would also be nicer to move the default from Nullable to Void, to make it more directly discoverable.