Parse Amalog into dicts

This makes the Prolog representation of an Amalog program more closely
match the native Amalog representation.  A program is just a term where
the keys are the predicate names, the values are a list of clauses.
Remove lexing code

I no longer see value in a stand alone lexer in Prolog.  For Prolog, it
makes more sense to have DCGs that perform real parsing.  I'll have
plenty of time to write lexers when I (eventually) support syntax
highlighting in editors.