cypher performance enhancements

rewrote pathimpl
replaced Iterator() with Iterator.empty
replaced List() with List.empty
replaced Seq() with Nil
replaced seq.size with alternative predicates
cypher performance part2

especially around paths
taking collection types into account
taking PathImpl into account
using Array to build up the node and rel collections in PathImpl
using collections.mutable.OpenHashMap in MutableMaps
Merge pull request #654 from jexp/cypher-performance

cypher performance enhancements