Dead code analysis for nested unused fields in contract adress types

[jubnzv]

The dead code analyzer could be improved to support recursive definitions like these:

transition tr4
  (
    x :
      ByStr20 with contract
        field y : ByStr20 with contract
          field unused : Uint32, (* should be reported *)
          field used : Int32
        end
      end
  )
  a <-& x.y;
  b <-& a.used
end

See: #1164 (review)