Internal adjustments

[alexis]

1. I don't think we got anything from the Base#ensure_symbol method.
   It would be more "duck-type oriented" and therefore more rubish if we just use ".to_sym" instead of all "ensure_symbol" invocations. I don't think we have to worry about descriptive error messages so much.

2. I don't like the fact_functor_obj hack (too much magic for me).
   I propose to define Base#private_fact?(fact) method instead.

[ipoval]

If we don't check that fact_name is String or Symbol it will allow to have facts like def_fact Object.new, right?, other than that I'm agree with the notices

[alexis]

Nope. Instances of Object don't respond to to_sym.