todo
updated utils, with correct murmur_hash, and cleaner IHashtbl
more uniform makefile
functorial version of Datalog.Logic;
the Symbols module disappears, because the functor now takes care of converting from/to symbols.
Default Logic instance where symbols are strings.
parser and datalog_cli use the Logic.Default module, with strings as symbols
Logic.S.db_premises now also returns the non-unit rule that is used to prove a fact