Provide method to determine typed data equality

Problem/Motivation

In quite some places we have to compare typed data, e.g. check uniqueness in add-item-to-list action.

Proposed resolution

Add a common piece of code for that.

Remaining tasks

* Get some guidance (probably @fago) for what already exists and where this code should go (probably typed_data module and later core).
* Collect issue to solve, e.g.
- references must be auto-dereferenced
- how to compare entities (e.g. - do we want to support that a cloned and modified entity is not equal that unmodified entity. how to compare two newly created entities. Maybe we want to compare their ->toString().)