[FIX] function fields: bound symbol methods of function field (type char) to the correct methods of the char field
[IMP] correctly bound _symbol_f and redifine self._symbol_set_char to keep the API
[IMP] move comment to the right line
[IMP] remove useless comment
[FIX] function fields: bound symbol methods of function field, type char, to the correct methods of the char field