Foreign Keys types are not mapped properly in relay

Let's create a simple relation:
```
table pen {
id uuid, primary key, unique
name text
}
```
```
table pig {
id uuid, primary key, unique
name text
pen uuid references(pen.id)
}
```

Now insert some pens. Now with relay, we might query `pen` it and get a node that looks like this:

```
"node": {
"id": "WzEsICJwdWJsaWMiLCAicGVuIiwgIjcwZmRkYTU1LTI2NGQtNGZhNy1hYTg2LWYzOWZhZTZmNGQwZiJd",
"name": "lxry pig pen"
}
```

Ok, all good so far. Now let's try insert a `pig`:

```
mutation MyMutation {
insert_pig_one(object: {name: "fatty", pen: "WzEsICJwdWJsaWMiLCAicGVuIiwgIjcwZmRkYTU1LTI2NGQtNGZhNy1hYTg2LWYzOWZhZTZmNGQwZiJd"}) {
name
id
}
}
```

Oh, shit. We get an error:
```
{
"errors": [
{
"extensions": {
"path": "$.selectionSet.insert_pig_one.args.object",
"code": "data-exception"
},
"message": "invalid input syntax for type uuid: \"WzEsICJwdWJsaWMiLCAicGVuIiwgIjcwZmRkYTU1LTI2NGQtNGZhNy1hYTg2LWYzOWZhZTZmNGQwZiJd\""
}
]
}
```

This is because it's expecting a uuid, but the relay endpoint never even exposes a uuid! The only way to even find out what it is would be to base-64 code that ID and then extract the uuid manually.

This seems like a pretty serious problem? No?

Reproduce the pen/pig schema, relay query, and insert_pig_one mutation shown in the issue. Trace how relay node IDs are exposed and how the pen foreign key is handled during insertion; done means the relay ID can be used for the foreign-key mutation without requiring manual decoding. No repository files or tests are named, and the issue has had no recent activity.

Written by the indexing model from the issue text.