New features and cleanup.

New features and cleanup.

New feature: support for rule retraction

  - rules are indexed by `Interpreter` and stored in a `Rule` instance.

  - REPL features to list rules, retract rule by it'd rule index.

New features: item retraction

  - this is an experimental feature, which is very inefficient.

Update python deps in readme

chart dumps and trace output use dyna-style double quoted strings and lower case
Booleans.

better output formatting for REPL/query