[strategies][length] make the default length result variant aware
[algorithms][length] change length return type to default_length_result<...>::type
[algorithms][perimeter] change perimeter return type to default_length_result<...>::type
[strategies][length (result)] eliminate result_of namespaces
[algorithms][length] replace result_of::length by default_length_result
[algorithms][perimeter] replace result_of::length by default_length_result
Merge pull request #86 from mkaravel/feature/length_perimeter_result

Feature/length perimeter result