Refactor type declaration, parents, interfaces to lang.ast.Type instances

[thekid]

Refactoring

This pull request changes uses lang.ast.Type subclasses in the following places:

TypeDeclaration implementations

+ use lang\ast\Type;
@@ ... @@
  abstract class TypeDeclaration extends Annotated {
-   public string $name;
+   public ?Type $name;
  }
@@ ... @@
  class ClassDeclaration extends TypeDeclaration {
-   public ?string $parent;
-   public array<string> $implements;
+   public ?Type $parent;
+   public array<Type> $implements;
}
@@ ... @@
  class InterfaceDeclaration extends TypeDeclaration {
-   public array<string> $parents;
+   public array<Type> $parents;
}
@@ ... @@
  class EnumDeclaration extends TypeDeclaration {
-   public array<string> $implements;
+   public array<Type> $implements;
}

NewExpression

- use lang\ast\Node;
+ use lang\ast\{Node, Type};
@@ ... @@
  class NewExpression extends Node {
-   public string|Node $type;
+   public Type $type;
  }

IsGeneric

  class IsGeneric extends Type {
-   public string $base;
+   public Type $base;
  }

It also introduces the lang.types.IsExpression class to be used for new $expr / new ($expr) cases.

BC break

This introduces as BC break (the XP compiler test suite will fail with Compile error (Cannot use 'self' as class name, as it is reserved), for example), and will thus result in a new major version (9.0 at the time of writing).