[lit-form] make model API not depend on form binder

[platosha]

In few cases, the current models' API implementation requires direct access to the respective binder node:

• AbstractModel::valueOf() - alternative: binderNode.value
• AbstractModel::toString() - alternative: String(binderNode.value)
• ArrayModel::[Symbol.iterator]() - no alternative yet

As part of introducing Object-based models, and extracting models into a separate package for wider use, it would be great to replace these APIs with some alternatives that do not require invoking binder nodes from models.

Let's deprecate these APIs and provide alternative API for iterating arrays in the binder node.

Documentation needs to be updated accordingly.