Will be used for testing occurrences in ZA/PDA.

No recursion, just iterations using stack.

So far only the PrefixRankedTree -> NPDA version existed. This commit
adds a version that constructs an NFTA for RankedTree.

For efficiency, let's implement runs of deterministic Z-Automata also
on linearised unranked tree.

Based on my testing with a tree constructed from sprotall.xml [1] the
run is ~15 % faster than a run over UnrankedTree.

[1]: https://research.cs.wisc.edu/niagara/data.html

also makes protected fields private

Closes #205

Closes #215

This new easy algorithm checks whether a regexp's language is an
infinite language.
Also adding an AQL test to compare with the same algorithm on finite
automata.