Requirements on parallel_scan body

Class `Body` implementing the concept of parallel_scan body must define:

- Body::Body( Body&, split );
  Splitting constructor. Split `b` so that `this` and `b` can accumulate separately
- Body::~Body();
  Destructor
- void Body::operator()( const Range& r, pre_scan_tag );
  Preprocess iterations for range `r`
- void Body::operator()( const Range& r, final_scan_tag );
  Do final processing for iterations of range `r`
- void Body::reverse_join( Body& a );
  Merge preprocessing state of `a` into `this`, where `a` was created earlier from `b` by b's splitting constructor

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