Iterates over resolved instances for given provider keys.

        :param keys: Provider keys
        :type keys: tuple
        :return: Iterator of resolved instances
        :rtype: generator

def iresolve(self, *keys):
        '''
        Iterates over resolved instances for given provider keys.

        :param keys: Provider keys
        :type keys: tuple
        :return: Iterator of resolved instances
        :rtype: generator
        '''
        for key in keys:
            missing = self.get_missing_deps(key)
            if missing:
                raise UnresolvableError("Missing dependencies for %s: %s" % (key, missing))

            provider = self._providers.get(key)
            if not provider:
                raise UnresolvableError("Provider does not exist for %s" % key)

            yield provider()