Rethink / -design Path

[gammelalf]

Currently Path represents a "path of joins" i.e. a step always corresponded to a join.

This means to properly support FieldProxy, mcf would have to do a lot of weird generic workarounds (if even possible)

A better solution might be to rethink what a Path represents. One idea:
A Path simply corresponds to the chain of field accesses used to construct it:

User.groups.owner.name
\_______________/ \__/
      path       field