TODO list for DT::Set, DT::Span, and DT::SpanSet

- implement as_list() in DT::SpanSet.pm

- discuss functions like 
    is_infinite
    is_empty 

- implement faster DT::Set::complement(), 
  using 'function composition'

- move all 'set-specific' code to Set::Infinite::_recurrence 

- functions like before(), after(), during() might be useful
  in DT::Span

- tests for DT::Span and DT::SpanSet 'duration()'
- duration() returns 'Inf' scalar - should be a DT::Duration::Infinite

- rewrite intersection of recurrences 'previous' function

