Introduce an intermediate level: abstract graph

[fehu]

Functors should map high-level expressions to an intermediate level, not to cypher fragments directly.

This layer should describe the paths (arrows) within an abstract graph.
Those paths should then be mapped to cypher expressions.

This layer will also serve as a base for introducing gremlin-like syntax in the future.

• Initial vertex selection
• 'Move to' relation from node
  • Outgoing
  • Incoming
• 'Move to' node from relation
  • Source
  • Target
• Select property