Ability to access parent model

[rossmacarthur]

It would be nice on a Model instance to be able to access the parent model instance. For example

from serde import Model, fields

class SubExample(Model):
    a = fields.Int()
    
    @property
    def parent_id(self):
        return self._parent.id

class Example(Model):
    id = fields.Int()
    b = fields.Nested(SubExample)


e = Example(id=10, b=SubExample(a=5))
assert e.b.parent_id == 10

[rossmacarthur]

Resolved in #83