ForeignField not compatible with BigAutoField
Fixed, thanks for the excellent report.
Description:

Column column_a in table B is a foreign key which references to primary_key a in table A. But if field type of primary_key a is BigAutoField, create table B will raise Exception(cann't add foreign key constraint).

How to reproduce the problem?

```python
import peewee as pw
from playhouse import db_url

db = db_url.connect('mysql+pool://root:root1234@localhost:3306/user')

class User(pw.Model):
    uid = pw.BigAutoField()
    class Meta:
        table_name = 'user'
        database = db


class UserSocial(pw.Model):
    uid = pw.ForeignKeyField(User, backref='userSocials', index=True, column_name='uid')
    class Meta:
        table_name = 'user_social'
        database = db

User.create_table()
UserSocial.create_table()

```

But if I change `BigAutoField` to `AutoField`, create_table will succeed. Or if I still want to use BigInt, I could update code like this

```python
import peewee as pw
from playhouse import db_url

db = db_url.connect('mysql+pool://root:root1234@localhost:3306/user')

class User(pw.Model):
    uid = pw.BigIntegerField(primary_key=True)
    uid.field_type = 'BigAuto'
    class Meta:
        table_name = 'user'
        database = db


class UserSocial(pw.Model):
    uid = pw.BigIntegerField()
    class Meta:
        table_name = 'user_social'
        database = db
        constraints = [pw.SQL('FOREIGN KEY(uid) '
                              'REFERENCES `user`(`uid`)')]

User.create_table()
UserSocial.create_table()

```