Lazy FstProperties + num_{input,output}_epsilons

[Garvys]

Goal1 : Make the fst.properties() method behave similary in rustfst and in openfst. At the moment, in rustfst all the properties are re-computed each time the properties method is called. To do that, we need to iterate over the whole graph which is slow.
However, in openfst the properties are stored in the Fst object and modified each time a mutable operation is performed.

Goal2 : Maje the fst.num_input_epsilons() and fst.num_output_epsilons() methods behave similarly in rustfst and in openfst i.e by maintaining internal counters instead of iterating through all the Trs.

Added

• Add compute_and_update_properties() method to MutableFst trait to compute the properties verified by the Fst and update the internal property bits.

Changed

• Now the fst.properties() method returns the stored property bits instead of computing all the verified properties.
• Now tr_sort no longer need a sorting closure but a struct implementing TrCompare.
  • ilabel_compare -> ILabelCompare
  • olabel_compare -> OLabelCompare
• num_input_epsilons and num_output_epsilons are now required methods of the CoreFst trait instead of provided.
• num_input_epsilons and num_output_epsilons are now much faster as they leverage internal counters instead of iterating through the Trs.