todos, and pp_plan function
deep copy of DB implemented
stupid error fixed
parser for queries (select-like syntax);
datalog_cli provides basic support for queries
first implementation of Datalog.Query (not efficient yet)
Reord is really Project, in the relational calculus of Datalog.Query
in Query, tables are now hashtables, for unicity checks
more accurate printing of plans
specialized ProjectJoin operator; also project+product
Datalog.Query features negated literals in queries
Merge branch 'query', with snapshot relational queries
minor fixes in signatures, after the merge