orm: support relationships

[terrablue]

[terrablue]

idea:

import relation from "primate/orm/relation";
import key from "primate/orm/key";
import p from "pema";
import User from "#store/User";
import Comment from "#store/Comment";

export default store({
  id: key.primary(p.u16), // u16 = 4 bytes = maps to `bigint` (since pg numbers are signed) or `serial` in postgres
  title: p.string,
  author_id: key.foreign(p.u16),
  author: relation.belongsTo(User, "author_id"),
  comments: relation.many(Comment, "post_id"),
});