Prototype: Complex attributes

[trask]

See #7621

[trask]

Suggested change

	static AttributeKey<List<Value<?>>> arrayKey(String key) {
	return InternalAttributeKeyImpl.create(key, AttributeType.ARRAY);
	}
	/** Returns a new AttributeKey for map valued attributes. */
	static AttributeKey<Attributes> mapKey(String key) {
	static AttributeKey<AttributeList> arrayKey(String key) {
	return InternalAttributeKeyImpl.create(key, AttributeType.ARRAY);
	}
	/** Returns a new AttributeKey for map valued attributes. */
	static AttributeKey<AttributeMap> mapKey(String key) {