Replace the visitor to extract elements

[Baptouuuu]

Problem

The current Elements visitor returns a Set meaning that it cannot be streamed as every element needs to be kept in memory. This prevents working on large documents that can't fit in memory.

Solution

• Add a new visitor with the signature All::elements(non-empty-string)(Node): Sequence<Element>
• Deprecate Elements visitor

The important part of this new visitor is that it returns a Sequence that can be lazy.