resolve symbol role conflict

[athos]

Each symbol has roles among special, macro, var, local or symbol (for now). And we now temporarily assume that every symbol has only a single role.
Taking into account more complicated situations (e.g. where a symbol would be put into multiple places through macro expansion), we should allow symbols to take more than a role. To this end, the extractor must be equipped with a kind of conflict resolusion mechanism.

[athos]

This issue has been exported to #5.