more coherent lookup extraction functions

reorganization of code to support DFG otuput

* Pure: 'advanced' translation functions use Context.generic instead of just theory;

declare defn rules;

all styles now reset to defaults first, i.e. the document may switch styles back and forth; added macro \isachardefaults for \chardef defaults; renamed \isakeywordcharunderscore to \isacharunderscorekeyword; added isabellestyle{default};

export meta_rewrite_rule; tuned;