Tabling

[gares]

following the prototype at https://github.com/LPCIC/elpi/tree/tabling/toy/ we add SLG exploration of the search space.

TODO:

• trail using persistent data structures (search tree heap is self contained, easy to have multiple ones)
  • perf seem ok, little more memory is used in general but nothing huge
• enrich stack/choice points with SLG info
  • this gives a little slowdown, since all choice frames are a bit larger
• concrete syntax to table a predicate
• index for the table
• abstraction for term (canonical term to be indexed, root of SLG tree)
• SLG loop
• abstraction API for constraints