Optimize simple for-like commands

[KaruroChori]

At the moment, for & for-props in both element and attribute versions are computationally heavy to support slices over sorted and filtered content.
An alternative execution path should be provided when no sorting nor filtering is needed.
Considering that such operations are performed externally for external sources, it does make sense to have a lightweight execution of "simple" cycles.

• parsing optimization (single pass detection of all children)
• data source optimization (no intermediate data is sorting is not needed)