A few minor tweaks:
A few minor tweaks:

- API for `BDD::sat_one()`: return assignment HashMap from user-visible
  function, but build it in mutable style internally. This helps to avoid
  the user error where they forget to clear the HashMap.

- Test of `sat_one()` in random-expression test: compute whether we
  actually expect the expression to be satisfiable or not during the
  exhaustive input-state loop, and compare to SAT result.

cfallin/boolean_expression,cfallin/boolean_expression