[distance] perform reverse-dispatch for default strategy
[strategies] implement default_comparable_distance_result class
[distance] re-implement the default strategy type in free function distance(g1, g2); remove inclusion of unused headers;
[comparable distance] simplify the definition of the default comparable strategy using the class detail::distance::default_strategy
[strategies][distance] re-implement the dfeault distance result using the default strategy from detail::distance::default_strategy