Automatic foreign key handling
Currently, Norm models are deliberately very close to the generated table schema. To define a one-to-many relation, you just mark a field with `fk` pragma and reference another model in it.

However, key benefit of having an ORM is that it gives you a nicer interface to your data, so you don't want a field that contains an id of the referenced object like in a DB, you want the referenced object itself. Currently, to achieve that, you should write a parser manually (see https://github.com/moigagoo/norm-sample-webapp/blob/develop/src/models.nim#L19)

To make Norm a more useful ORM, we should handle foreign keys automatically. This:
```
type
  User = object
    name: string
  Pet = object
    owner: User
```
should be equivalent to this:
```
type
  User = object
    name: string
 Pet = object
    owner {.
      fk: User,
      dbCol: "ownerId",
      dbType: "INTEGER NOT NULL",
      parseIt: User.getOne(it.i.int),
      formatIt: ?it.id
    .}: User
```