Update non-core module "stack"

[Chris Hillery]

Update non-core module "stack". Refer to the Zorba 3.0 Module Update Checklist at http://my.zorba-xquery.com/dokuwiki/doku.php?id=module_update .

This minimally means renaming the module to the "http://zorba.io/modules/stack" namespace, ensuring the documentation is accurate and up to standards, and moving the module contents out of unnecessary com/zorba-xquery/ subdirectories.

[Chris Hillery (ceejatec)]

Per Matthias, it makes sense to update this module to be JSONiq-friendly as well. Currently it only supports node items (which are only XML), but there's no real reason to limit it this way.

All the functions that accept or return "node()" should be changed to accept/return "structured-item()". (structured-item() means "node, JSON object, or JSON array".)

Also, currently the stack module implementation is using node-specific functions in the DML libraries. Those functions are being deprecated and replaced with more general item-based functions. For instance, instead of dml:delete-nodes-first(), use the dml:delete() function which takes an item() rather than a node() argument.