[algorithms][distance] eliminate result_of namespaces; replace
result_of::distance by distance_result;
[algorithms][comparable distance] make comparable distance algorithm
variant-aware
[strategies][distance (result)] polish code a bit;
eliminate result_of namespaces; replace result_of::distance by
distance_result; add support for comparable distances (i.e., add
specializations for default_comparable_strategy);
qualify occurences of variant with boost:: ;
fix bug: when resolving strategies, pass actual geometries instead
of their point types (important in order to return the actual/correct
strategy return type);
[strategies][comparable distance (result)] implement variant-aware
comparable distance result
[strategies][comaprable distance] add default_comparable_strategy:
this is a placeholder for comparable distance strategies used for
resolving strategies during the resolve variant stages
[test][algorithms][distance] replace result_of::distance by distance_result
[test][algorithms][comparable distance] add tests for variant support