Check for cycles in context defs - any length

[Rune Zedeler]

I have made a check for cycles at any length - pushed it to dev/rune.
See files context-def.cc and context-def.hh

Main problem is that Context_def::path_to_acceptable_context cannot 
anymore be marked as const because it needs to flag the contexts that it 
visits.
Is this ok?

I still haven't made convert-ly-rules nor removed the inner-contexts. I 
will do this as soon as this gets accepted.

-Rune