Updated valuations and paths API.

[daemontus]

This pull request resolves BddValuation and BddParitalValuation API deficiencies described in #22. Specifically, it includes following improvements:

• Implements a check for whether a Bdd is a single clause/valuation.
• Completely redesigned path/valuation iterators. Now it is possible to iterate just paths, or valuations of a single path.
• Valuation introspection utilities: first/last path/valuation, most positive/negative valuation, most free/fixed path.
• A tutorial module which explains most of this.

[codecov[bot]]

Codecov Report

Merging #25 (89df5ca) into master (d41d348) will decrease coverage by 0.56%. The diff coverage is 93.17%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
- Coverage   95.31%   94.74%   -0.57%     
==========================================
  Files          21       24       +3     
  Lines        1493     1866     +373     
==========================================
+ Hits         1423     1768     +345     
- Misses         70       98      +28     

Impacted Files	Coverage Δ
src/_impl_bdd_satisfying_valuations.rs	94.87% <83.33%> (-2.69%)	:arrow_down:
src/_impl_bdd_valuation.rs	79.64% <86.84%> (-0.36%)	:arrow_down:
src/_impl_bdd/_impl_valuation_utils.rs	92.17% <92.17%> (ø)
src/_impl_bdd_path_iterator.rs	94.20% <94.20%> (ø)
src/_impl_bdd/_impl_util.rs	88.03% <95.45%> (+1.97%)	:arrow_up:
src/_impl_bdd_partial_valuation.rs	98.46% <100.00%> (+0.58%)	:arrow_up:
src/_impl_bdd_variable_set.rs	94.20% <100.00%> (ø)
src/_impl_iterator_valuations_of_clause.rs	100.00% <100.00%> (ø)
src/_test_bdd/_test_bdd_logic_fuzzing.rs	100.00% <100.00%> (ø)
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update d41d348...89df5ca. Read the comment docs.