Add type-specific get_children

get_children is elegant and flexible and slow.

    def get_children(self):
        for field in self._astroid_fields:
            attr = getattr(self, field)
            if attr is None:
                continue
            if isinstance(attr, (list, tuple)):
                for elt in attr:
                    yield elt
            else:
                yield attr

It iterates over a list, dynamically accesses attributes, does null
checks, and does type checking. This function gets called a lot, and
all that extra work is a real drag on performance.

In most cases there isn't any need to do any of these checks. Take an
Assign node for instance:

    def get_children(self):
        for elt in self.targets:
            yield elt

        yield self.value

It's known in advance that Assign nodes have a list of targets and a
value, so just yield those without checking anything.

Author: nickdrozd